This PDF file contains the front matter associated with SPIE Proceedings Volume 11717, including the Title Page, Copyright information, and Table of Contents.

Offner imaging spectrometer consists of a convex grating and two concave mirrors. The concentric characteristics of the optical structure make it have the advantages of large relative aperture, small distortion and compact structure. In order to reduce the alignment difficulty of the Offner imaging spectrometer and improve the efficiency, a fast alignment method of Offner imaging spectrometer is presented in this paper based on the concentric characteristic and spherical autostigmatic method. Firstly, the spherical autostigmatic device is built, which can generate point source, and when the point source is located at the spherical mirror’s center of curvature (CoC), its reflection image point and the point source coincide. By measuring the distance between the reflection image point and the point source, the position deviation of the spherical mirror’s CoC can be determined. The Offner imaging spectrometer is completed with this device by locating the CoC of its primary mirror, convex grating and tertiary mirrors. The results show that location error of the two off-axis concave mirrors’s CoC can be controlled within 10μm, and the imaging performance of the imaging spectral system is satisfied. Compared with the existing method, this method has the advantages such as easy to operate, low cost and fast alignment.

In this paper, we investigate that the effects of weather, turbulence and pointing errors on laser beams, and establish a joint channel statistical model, then derive the performance parameters of free space optical communication system based on OOK modulation, such as average bit error rate, average channel capacity and outage probability, and finally derive their new expressions by using Meijer’s G function and H function. For an atmospheric laser communication system with a wavelength of 1550 nm and a link length of 1 km, the performance parameters of the system are simulated and analyzed. The results show that when the standard deviation of jitter is small, the communication system is obviously affected by turbulence, and the performance of communication system with different turbulence can be significantly improved by the aperture average effect; aperture averaging can also significantly compensate for the deterioration of the communication system performance due to the decrease of atmospheric visibility; when the standard deviation of jitter is large, the system performance is limited by the average aperture effect.

This study presents the hydrodynamic simulation for high energy large-size Nd:YAG liquid-cooled laser amplifier. Based on hydrodynamic equations, the heat transfer coefficient as a function of cooling fluid velocity is derived. The velocity of cooling fluid could be chosen as 0.8m/s by considering critical Reynolds numbers for laminar and turbulent flows. In order to assess the uniformity of the cooling fluid, a four-channel fluid model is established. The uniformity of four D₂O fluid channel in gain region are 98.5%, 98.7%, 98.5%, 98.6%, respectively. Besides, A simple Nd:YAG heat transfer model is built for assessing the cooling capacity of fluid, which is based on the calculation of heat transfer coefficients equation. The temperature differences of central and marginal gain medium planes is 3.451K and 1.951K, respectively, which is close to 3.255K and 1.778K calculated in the heat-fluid-solid coupling model. Based on the heat fluid-solid coupling model, the total wavefront aberration is 0.266λ cooled by D₂O. Finally, another cooling fluid FC770 has been compared with D₂O. In terms of fluid uniformity, the uniformity of four FC770 fluid channels in the gain region are 98.5%, 98.7%, 98.5%, 98.6%, respectively, which is close to D₂O. In terms of heat effect, the heat transfer coefficients of D2O is larger than FC770. And total wavefront aberration of Nd:YAG cooled by FC770 is 0.840λ, which is larger than D₂O. Eventually, the thermally induced wavefront aberrations of D2O and FC770 are 0.0475λ and 0.6092λ, respectively. The calculated results showed that D₂O is a better cooling fluid than FC770.

In the process of underwater lidar wake detection, the multipath effect leads to pulse stretching of the echo signal, which is a distinguishing feature to distinguish the wake echo signal. In order to explore the factors that affect the pulse stretching of the echo signal, this paper convolves the instantaneous echo energy of the bubbles at different distance layers with the transmitted pulse, and establishes a distance layer summation model(DLSM) of underwater bubbles. This paper analyzes the effect of bubble density on the backscattering coefficient of the bubbles and the pulse width of the echo signal, and introduces the attenuation length of the water and the multipath effect. The experimental results show that when the attenuation length increases from 1.2 to 2.0, the half-peak width of the echoes of the bubbles increases by 1.3ns. However, when the attenuation length increases from 1.5 to 2.2, the echo pulse width of the strong backscatter target decreases by 0.5ns. The thickness of the bubbles increases from 2cm to 5cm, the peak shifts by 0.4ns, and the echo pulse width increases by 0.6ns. The simulation model and experimental results provide an effective basis for distinguishing ship wakes and strong backscattering targets, and have an important role in improving wake detection and recognition capabilities.

Laser shock processing (LSP) is a surface hardening technology by using plasma shock waves, which can improve the mechanical properties of material and extend the service life significantly. This work introduced the mechanism of LSP briefly, the analysis of the effect of LSP on the surface morphology, microstructure of material and its related laws are summarized with the aspect from aluminum alloy, titanium alloy and superalloy. The results show that the strong plastic deformation would be appeared in the near-surface of material due to the laser-generated stress effect, resulting the change of surface roughness and the microstructure evolution. At the same time, high-density dislocation walls, dislocation tangles and mechanical twins are generated on the surface, which are accompanied by changes in grain refinement and increased grain boundaries, thereby significantly improve the mechanical properties of material. The convex structure can be found on different material surfaces after LSP, which is micro-scale reverse deformation caused by the free plastic flow of materials under laser shock. And the high pressure of shock waves can lead to destructive dimples on the material surface, and affect its service performance. Aiming at the characteristics of different material, this work comprehensively compares the differences in the surface morphology and microstructure changes of the alloys caused by LSP, and makes a detailed analysis and discussion. This work is helpful for researchers to further understand the high strain rate dynamic plastic deformation mechanism of material treated by LSP, and promote the development of LSP.

We introduce some physical optical effects into a laser to construct a novel laser system. Using a twisted fiber as an optical delay feedback external cavity based on birefringence of twisted fiber, we present a novel delay feedback semiconductor laser system with the twisted fiber effect and establish its physical model, in which the effect of the twisted fiber birefringence is included to lead to dynamics in the laser system. We give linear analysis of stability of the influences of delayed phase and rotation angle of twisted fiber on the oscillation characteristics and behavior of the laser. And we give quantitative analysis of both delayed phase and rotation angle, which resulted from the physical effect of twisted fiber, and analyze to how to alter the relaxation oscillation or dynamics state. Because the twisted fiber can change the direction of light polarization and produce phase shift after light passing through the twisted fiber, the laser system is sensitive to phase change and polarization while some dynamic changes of the laser can be caused and excited. We discovery theoretical that the delayed phase and the rotation angle resulted from the twisted fiber can alter the relaxation oscillation and dynamics state. And the fiber parameter’s variety can shift the delayed phase, the rotation angle, the delayed time, and the optical feedback level, so the laser’s behavior will vary. We find that the scaling of the transmitting time-delay of fiber and the laser internal timescale creates an incorporation to induce various dynamical scenarios.

To meet the high efficient and pollution-free paint striping requirement of high-speed railway carriages, a nanosecond fiber laser paint striping experiment is proposed. The influence of the pulse width and the laser power on the paint removed thickness and effect are studied, the paint removal mechanism is analyzed and the paint removal parameters are optimized. The result shows that, the main mechanism of laser paint striping is the thermal ablation effect and the vibration effect, with the pulse width increase, the paint removal thickness will increase firstly and then decrease slightly, which is caused by the combination of the increased thermal effect and the decreased vibration effect. With the increase of laser power, the paint removal thickness will increase, and when the laser power becomes too large, the substrate is oxidized and turns black. After the parameter optimization, the paint removal efficiency reaches 1.4m2/h and the surface of the sample is undamaged, obviously the laser paint striping can meet the paint removal requirements of the high-speed railway carriages.

Aiming at the problem that atmospheric laser communication is easily affected by atmospheric turbulence, which will lead to the degradation of communication quality, RNN and LSTM were established based on deep learning to predict refractive index structure constant, one of the most important parameters of atmospheric turbulence. Based on it, reference for the selection of atmospheric laser communication channels can be provided to avoid waste of channel resources. Three statistical values average absolute error, root mean square error and correlation coefficient were used to analyze the prediction results. The results showed that both RNN and LSTM can predict very well under medium and strong turbulence. The correlation coefficient between the predicted data and the original data were 67.37% and 96.17%.

An 808 nm LD end-pumped trapezoidal 1.064μm Nd:YAG laser was demonstrated in this paper. The advantage of using a trapezoidal crystal was that under the same volume of the gain medium, the length of the gain medium was significantly increased, which increased the output power. The maximum continuous wave average power of 2.16W was obtained with the pump power of 30.5W, and the slope efficiency was 13.75%. After adding an acousto-optic Q switch with selective polarization function in the cavity, the pulse energy of 1.05 mJ, the pulse width of 80.7 ns at the PRF of 1 kHz was obtained. And the output laser was s-polarized. The beam quality factor was measured as M²~2.95.

The deposition of laser energy will ionize the air and generate high-temperature and low-density plasma hot core. The interaction of laser-induced plasma with quiescent air and shock waves can change the flow structure, which will change the local pressure. The evolution of the plasma hot core in the quiescent air and the interaction of the hot core and the normal shock at Mach number 1.75 are numerically simulated. The results show that the hot core evolves into a threelobed structure in quiescent air due to the Richtmyer-Meshkov instability; The baroclinic pressure gradient caused by the interaction of shock wave and the hot core boundary leads to the splitting of the hot core into two vortex rings, which are the key reason of the change of the flow field caused by laser.

Defined as a visual inhomogeneity, Mura can cause seriously unpleasant feelings and that’s why it needs to be inspected. Band Mura, which has a large area, is particularly difficult to be detected because of its irregular shape and size as well as its low contrast. So we propose a UADD-GAN model to detect band Mura in this work. Consisting of a proposed UADD generator and a discriminator, the model is trained with some normal samples, after which the generator is able to simulate the distribution of normal samples. During training, the generator takes normal images as inputs and output their reconstructions, while the discriminator receives images and determines whether they’re true or fake, defiantly helping the generator to perform reconstructions better. The symmetric structure and operation of skipadding make it easy for the UADD generator to reconstruct the normal samples well. On the contrary, the generator performs worse in the reconstruction of Mura samples so that we can distinguish them from the normal ones. In addition, we use multiple feature layers of the discriminator for supervision instead of using only the classification layer, helping the generator to reconstruct normal samples better. We’ve conducted experiments of MURA data sets with different proportion and our research indicates that the proposed model surpass other state of the art methods.

Photon tunneling effect and transmittance are investigated through the composite barrier of both positive index material and negative index medium with sinusoidal interface. Based on optical admittance and characteristic matrixes, the calculation method of transmittance is simplified. The seven-layer structure is proved to be effective in enhancing the photon tunneling. The transmittance resonance with the negative refractive index and amplitude of sinusoidal interface is analyzed. This method to generate transmittance resonance is significant when the width or thickness of the structure is restrictive.

Warm laser shock processing (WLSP) is an innovative method of surface modification to obviously improve materials’ surface strength stability and service performance in elevated temperature, due to the thermal-mechanical interaction of Laser shock processing, Dynamic strain aging and Dynamic precipitation. In the work, the fundamental mechanism and technological difficulty of WLSP are summarized. The technology can’t be applied in industry ripely for lacking new absorbed layer and confined layer, the method of controlling temperature precisely. The research progress of WLSP in the surface modification is analyzed respectively from the surface’s profiles, the residual stress’ amplitude and stability, the microstructure evolution, and service performance et al. in modified materials. Elevated temperature and severe plastic deformation are the key factors of WLSP, and nano precipitates and high-density dislocations induced in the process are the basic reasons for the thermal stability of residual stress and microstructure. In addition, after reviewing the influence of severe plastic deformation, the pinning effect of nanoscale precipitation and high dislocation on the materials’ structure, the problems of the technology are pointed out, providing a theoretical basis in the field.

In this paper, based on the Lorentz equation and Maxwell's equations the single electron acceleration model was established, we use MATLAB to study the electron dynamics and radiation emission in the field of a relativistic intense laser pulse. In the spatial distribution of the electron appear "ramifications", and the intensity of electron radiation angle of peak time spectrum showed that, when the beam waist radius from 7λ₀ to 4λ₀, the main peak change from multiple into a single gradually, from 4λ₀ the main peak symmetric bimodal pattern is obvious damage, and the intensity of the main peak is mainly on the left side. The radiation peak is positively correlated with the beam waist radius while the pulse width is just the opposite. Combining with the electronic motion images, we find that the change of the growth trend also occurs at 4λ₀ and 7λ₀. It marks that, it's possibly that there is an intermediate state between the tight-focused and the non-tight-focused.

AlGaAs is one of the most promising integrated quantum photonics material. It has second order nonlinear coefficient which could support spontaneous down conversion (SPDC) process. The phase matching requirement could be achieve by specific designed Bragg reflection waveguide (BRW). In this paper, AlGaAs BRW was designed and simulated. Wavelength relationship of type II SPDC’ three photons were calculated. Epitaxy thickness and composition variation effects were also calculated.

Metamaterials has become a hot research field in recent years due to its unique artificial electromagnetic proper- ties. It has the potential to realize high-performance temperature sensors because its resonance features are highly dependent on the surrounding environment. In this paper, we designed a metamaterial sensor with two-layer grating structure and studied the relationship between its resonance dip and the incident angle of electromagnetic wave. By fixing the incident angle at a proper value, we realized a highly sensitive temperature sensor. The measured temperature sensitivity and limit of detection are about 0.766%/°C and 0.154°C, respectively.

Within the frame of electrodynamic and nonlinearly Thomson scattering, we study the initial phase-sensitivity with respect to spatial characteristics of a static electron irradiated by Guassian linearly polarized few-cycle laser pulse for different beam waists. With the combination and comparison of the electron's real-time motion and radiation conditions, the initial phase-sensitivity turns out with inverse proportional relation to the beam waist where the radiation’s transverse shift of varied initial phases is irregular in the non-tightly focused case. Furthermore, by changing the initial phases, the petal-like full spatial radiation pattern changes from 4 unstable pieces to 2 constant pieces and the radiated energy shift slows down with the beam waist evolving from b₀ = 1 to b₀ = 10. Finally, with initial phase 𝜙0 increasing from 0 to π we discover an interesting phenomenon for the first time, where the normalized maximal radiated power and corresponding azimuth angle fluctuate in the form of ripples and ladders in tightly-focused case b₀ = 1, in contrast to the perfectly symmetry with respect to φ₀ = 0.5π for the maximal radiated power in non-tightly focused case b₀ = 10.

We study control-chaos of a dual-ring erbium-doped fiber laser using an anti-parameter pair modulation. And we study this control-chaos method via the inverse modulation of the gain parameters of two key physical parameters of the dualring erbium-doped fiber laser. In terms of physical mechanism, the optical effects of alternating excitation or alternating attenuation of two gain parameters of the laser are found. The control of chaos laser and its new nonlinear dynamic changes are derived. In the control mechanism, the double-parameter reverse modulation pair is found where the laser behavior is very sensitive to the anti-parameter pair modulation because this reverse action modulation combines with induction control dynamic changes. The laser can be controlled to a single-periodic state, a double-periodic state, a threeperiodic state, a four-periodic state, a five-periodic state, a six-periodic state, a seven-periodic state and other highperiodic states. Double-dynamics, frequency controlled-locking and semi-frequency controlled-locking regions are found. The laser exhibits a very imaginative dynamic behavior that has never been seen before, which has important reference value for the study of chaos dynamics, laser technology and control technology.

Publisher’s Note: This paper, originally published on 2 December 2020, was withdrawn per author request.

Using the theory of single free electron in the form of Thomson scattering, through theoretical calculations and computer simulations, the properties of radiation pulses generated by ultrashort laser pulses are studied. Calculations show that in this case, the pulse width of the electron’s maximum radiation pulse reaches on the order of attosecond. We focus on nonlinear Thomson scattering. Under the condition of circularly polarized tightly focused laser pulses (b₀ = 3λ₀), we change the intensity of the incident laser. Generally, under the condition of relativistic laser intensity, keep the beam waist radius not changing, the larger the incident laser’s peak amplitude (a₀), the larger the maximum of electron radiation power. After that, we focused on the temporal characters at the angle when the electron radiated power was at its maximum. At some special incident laser’s peak amplitude (a₀), the change of pulse width in electron radiation power has some rules. We divide the change of pulse width into three categories according to the number of maximal values in the angular distribution of the radiation energy, and discuss the regularity of them separately.

We experimentally investigated the two-mode coexistence phenomenon in a 1550-nm Vertical-Cavity Surface-Emitting Laser (1550-nm VCSEL) subject to variable polarization optical feedback (VPOF). First, for the fixed feedback strength, through appropriately adjusting the polarization angel of the optical feedback and bias current, the two-mode coexistence including parallel linear polarized mode and vertical linear polarized mode can be found when the bias current is set about 6.50 mA. Meanwhile, a mapping of polarization mode suppression ratio (PMSR) between two orthogonal linear polarization modes is portrayed in the parameter space of the bias current and polarization angel of optical feedback. Second, for a given bias current, with the increase of feedback strength, the two-mode coexistence region is widely expanded for continually varied polarization angel of optical feedback. At the same time, a mapping of PMSR is also presented in the parameter space of feedback strength and polarization angel. It can be seen that reasonable feedback strength and polarization angle are more conducive to expand the two-mode coexistence parameter region.

High-power fiber laser welding has proven to be the most effective high-speed automatic welding technology. It is generally believed that the keyhole structure contains a large amount of welding quality information. Its behavior and instability are one of the causes of welding quality defects, especially the porosity. In this paper, we propose a fast-online detection method for high-power fiber laser welding. In view of the characteristics that the behavior and stability of the keyhole have an important impact on the welding quality, the real-time image of the keyhole is taken during the welding process, and the image is binarized through the adaptive threshold selection, and the maximum connecting area is selected to quickly get the contour of the keyhole. By combining the global convexity of the keyhole contour with the local angularity on the micro level, the support vector machine model is trained as the input data. Experiments show that the classifier has high accuracy. The combination of these features can characterize the pore defects, quickly and real-time find potential pores, reduce the cost and time of later detection, and explain abnormal metal flows in and out of keyholes when defects occur.

The wavelength tuning interferometer is widely used in the detection of large-aperture optical components. Limited by the accuracy of wavelength modulation, the calculation of phase shift is difficult to meet the accuracy requirements. At present, the least squares iterative algorithm is often used to solve the phase. However, this algorithm needs to accurately calibrate the cavity length first, and the calculation error of the phase shift amount in some specific intervals is large, especially when there is a contrast difference between the interferograms, the detection accuracy is even more difficult to meet the requirements. This paper proposes a method to analyze the phase shift directly from the interferogram, using an improved iterative algorithm to calculate the wave-front phase, and proposes a self-applicable segmentation processing method for the phase calculation in a specific interval to meet the system accuracy and robustness requirements. The validity of the proposed method is verified by the modeling method, and the existing wavelength-tunable laser interferometer is used to test the sample. The repeatability of PV value and RMS value are λ⁄50 and λ⁄100 respectively.

This paper proposes a GaN-based light-emitting diode based on an n-i-p terminal quantum barrier structure that is easy to implement in epitaxy, which can enhance electron confinement and improve hole injection efficiency. Existing GaNbased light-emitting diodes, due to the electron blocking layer (EBL) formed by inserting a wide band gap AlGaN material between the active region and the p-type hole injection layer, cannot effectively confine electrons, and also suppresses multiple quantum wells (MQWs) and Hole injection in the p-type region caused by the polarization field in the EBL. In order to improve the performance of GaN-based LEDs, better suppress electron leakage in the active region and increase the hole injection efficiency in the p-type region, a new design of the terminal quantum barrier was carried out, specifically n-GaN n-GaN (1×10¹⁸cm^-3)-i-GaN-p-GaN (5×10¹⁸cm^-3) terminal quantum barrier structure, the thickness of the n, i and p layers in the barrier structure are 4 nm, 5 nm and 4 nm, respectively. This paper uses simulation method to verify the terminal quantum barrier structure. The electrical and optical performance of the GaNbased MQW LED with conventional structure and the LED based on the n-i-p terminal quantum barrier structure were compared, including physical indicators such as output optical power, threshold voltage, and hole injection concentration. The simulation results show that, compared with the traditional structure, the LED with the nip terminal quantum barrier structure has higher output light power and lower threshold voltage, and significantly increases the hole concentration in the active area, which is more effective in suppressing Electronic leaks. The analysis results show that the strong reverse electrostatic field in the nip terminal quantum barrier structure can effectively enhance the electron confinement in the active region, suppress electron leakage and improve the p-type hole injection efficiency. In summary, it is shown that the nip terminal quantum barrier structure can be Effectively improve the performance of LEDs, showing better performance than conventional devices.

In this paper, a kind of photoacoustic detection system was established to measure the photoacoustic signals of glucose with different concentrations. In this system, the lateral model was used based on the optical parameters oscillator (OPO) pulsed laser and ultrasonic transducer. Meanwhile, the optimal absorption of 1064nm for the pulsed laser and the ultrasonic transducer with central frequency of 1MHz was used. The amplified time-resolved photoacoustic signals and photoacoustic peak-to-peak values of glucose aqueous solutions with different concentrations were obtained in the average of 128 times. In addition, the effect of detection distance on the photoacoustic detection of glucoses was experimentally investigated. The detection distance includes the vertical and the transverse distances between the photoacoustic source in the glucose aqueous solutions and the ultrasonic transducer were all considered. Moreover, the effect of detection distance on the prediction of glucose concentrations was also studied by using the linear fitting model. Experimental results show that for each glucose solution with different concentrations, the photoacoustic peak-to-peak value of glucose decreases in the form of exponent function with the increase of detection distance, and the peak point of glucose will shift toward right due to the increase of detection distance although the concentration isn’t changed. When the transverse distance between the photoacoustic source and the ultrasonic transducer increases, the photoacoustic peak-to-peak value of glucose linearly decreases. At the same time, the prediction accuracy of glucose concentration decreases with the increase of vertical and horizontal detection distances.

Mura is a phenomenon in which the displays have various uneven display defects. The band-shaped Mura has the characteristics of irregular shape and different sizes. And the new shapes and sizes of Mura may appear at any time during the inspection process. Therefore, traditional image algorithms are difficult to detect the band-shaped Mura anomaly. In response to the above problems, this paper proposes the Res-unetGAN network, which is an unsupervised anomaly detection method based on generative adversarial network. We design resnet50 as the encoding network of the generator to obtain the latent feature vectors. To improve the quality of reconstructed samples, we combine the skipconnection structure into the generator to guide the decoder. The discriminator is a convolutional neural network based on the Depthwise Separable Convolution. The purpose is to distinguish between normal samples and reconstructed samples, and form a game process with the generator. The network only needs normal screen samples during the training process. In the test, since the Mura sample has not been trained, the reconstruction error score of the Mura sample will be higher. After repeated experiments on the band-shaped Mura data set, the highest auc of 0.995 was obtained, which is better than several models for comparison.

The current three-dimensional (3D) imaging technique of dynamic scene has some problems, such as a large amount of data and high requirements for the hardware equipment. To remedy these defects, we propose a novel 3D dynamic compressive imaging system that combines structured-light 3D surface imaging with temporal ghost imaging (TGI). The system aims to acquire a 3D video through single exposure imaging, and then restore the 3D dynamic scene through the post-processing algorithm. Since a slow camera can be used to capture 3D video, the system has the advantages of low detector bandwidth and low data volume. Specifically, a fringe pattern with a frequency component is projected onto a moving object to obtain 3D information, and then image the object onto the digital metal micro-mirror device (DMD) for pixel-by-pixel optical encoding to achieve video compression, finally, the camera captures an image in a single exposure. To decode the image, the optical intensity correlation is calculated to obtain the temporal frames, which retain the deformed fringe patterns modulated by the object, then the 3D surface profile of the frame can be reconstructed through Fourier fringe analysis and phase-height mapping. The experiment verifies two types of motion scenes, translation and rotation, 15-frame 3D video is compressed in one image, that is, the frame rate of the camera is increased by 15 times, and the equivalent compression ratio is 6.67%.

Modulation and demodulation is an important key technology in optical communication. The design of modulation and demodulation will directly affect the transmission quality of the whole system. Therefore, the design of modulation and demodulation system is of great significance to the application of optical communication. In this paper, the PPM modulation and demodulation is designed as an independent module in FPGA, which is convenient for people to use. Through the oscilloscope, FPGA development board, PC hardware system and Quartus II, serial assistant, mdelsim software system to verify the design of the modulation and demodulation module function. PC serial port is used to send unmodulated digital signal to FPGA development board. FPGA modulates the data received by serial port. The modulated signal is divided into two channels. One channel is sent to PPM demodulation module. The demodulated signal and the other modulated signal are sent to PC through serial port. The oscilloscope is used to capture the sending and receiving pin signals. By comparing the waveform and serial port of the oscilloscope, two channels of waveform are compared Hand data to verify the design of the module. The main working process of PPM modulation module is to collect the original digital signal by sampling clock, and then to modulate it into PPM signal after buffering, encoding, storing and shifting. Ppm demodulation module collects PPM signal by sampling clock, caches, decodes, registers and shifts output, and finally demodulates the original signal. Through Modelsim simulation and oscilloscope hardware platform experiment, it shows that the PPM modulation and demodulation module designed in this paper has strong antiinterference ability, low bit error rate, convenient and flexible, and has good engineering application value. It realizes higher communication rate in optical communication system and improves the reliability of transmission system, which is very important for optical communication.

Pulse pressure measurement, as one of the major home health monitoring technologies, is of great significance for the assessment of blood pressure and recognition of lung cancer. Here we propose and experimentally demonstrate a noninvasive optical fiber pulse sensor, which comprises a simple structure in which a section of thin-core fiber is spliced into another single-mode fiber. Responses of fiber pulse sensor to the simulated pulsations are tested by using a piezoelectric transducer. And a silicone rubber device is designed to ensure that weak pulse signals are detected. The sensors with an inserted thin-core fiber length of 2-3 mm can display remarkable response to human pulse pressure. The signal waveforms detected are almost unaffected by the subject’s movement. It also possesses the advantages of high sensitivity, simple signal acquisition and processing, easy fabrication, and thus is an ideal candidate for replacing traditional electrical sensor.

Optical clocks have become a very important tool of investigating the characteristics of atoms or ions, for the state-of-art instability and uncertainty. Increasing the running time of an optical atomic clock is one critical issue that scientists are still working on. In this paper we report the improvement of the atomic chamber of a calcium beam optical clock. By using a combination of microtubes as the nozzle, we achieved a very high atomic flux with small divergence. The resonant atomic flux is about 1×10¹² to 10¹³ per second and the divergence is smaller than 30mrad. The experimental results show that the system is reasonable in design and meets the technical indicators.

As the local oscillator of the space optical clock, the ultra-stable laser determines the short-to-medium-term frequency stability of the space optical clock. Considering the space station’s restrictions on load weight and volume, as well as the impact of vibration and shock during launch, a tunable external cavity diode laser with small size, stable structure and no elastic adjustment device was developed. Optimized the design of the structure of the optical path board, developed small optical components, and developed a double-sided optical path system based on this. Experimental tests show that the free-running line width of the laser is about 175 kHz, which can run stably and reliably for a long time. At the same time, considering the deformation of the optical path substrate in the space microgravity environment, the topology optimization design of the optical board was carried out. Through mechanical simulation analysis, the maximum deformation of the optical path substrate under the influence of gravity is 0.43 μm, which initially meets the requirements of space applications.

The statistical characteristics of quantum noise stream cipher system is investigated. Simulation results shows that with the variance of the noise σ2 is changed from 0 to 0.3, the statistical parameter of signals decreases to the floor. When the variance of the noise is more than noise masking threshold, the statistical characteristic of signals is completely submerged in noise. Moreover, when the variance of the noise is equal to {0.001, 0.01}, PB =1 is unchanged with the increase of modulation order. Furthermore, when the variance of the noise is equal to {0.1, 0.2}, the correct probability for Bob PB decreases to a fixed value which is 0.99 and 0.79, respectively. Correspondingly, the correct probability for Eve PE decreases to 0 with the modulation order increasing. And the channel advantage for main channel over wiretap channel can be formed by combining proper modulation order with noise.

In this paper, a new mechanism of hot image formation introduced in frequency conversion process is introduced. Under the effect of second order nonlinear effect, defects can also cause downstream hot image, where the peak light intensity can reach several times of the average light intensity. The corresponding physical model was established, and the physical process and related laws of the upstream defect induced optical field enhancement under the second order nonlinear action were studied by theoretical analysis and numerical simulation. It is found that three wavelengths of laser will form corresponding hot images at different positions downstream of the nonlinear crystal. The effects of defect size and conversion efficiency on hot image strength are further studied. Finally it is found that the generation of hot image of triple frequency can be effectively inhibited by increasing the spacing between two crystals. This research can provide an important reference for beam quality control of final optical systems of high power laser deriver.

This paper demonstrates the influence of pulse width on the full space characteristics of electron motion and radiation distribution from a single electron driven by a linearly polarized femtosecond tightly focused gaussian laser pulse (FTFGLP). The laser-accelerated electron model is created by the interaction between a tightly focused linearly polarized femtosecond gaussian laser pulse and an electron. Driven by the electromagnetic field produced by laser pulse, the electron produces relativistic oscillation and the corresponding full spatial radiation. Through numerical calculation, it is concluded that the space characteristics of relative electron changes firstly and then tends to be stable. In detail, with the increase of plus width, the amplitude of electronic motion tends to a constant value, and there is an optimal value of the change of drift distance that tends to be stable. Moreover, due to the increase of plus width, the phase variation of electron motion in the characteristics of electron space radiation distribution intensifies, and the number of branch gradually increases, but the Angle of the maximum branch is basically unchanged. The characteristics is of great use in some real experiments of linearly polarized laser plus.

Based on the insulator to metal transition (IMT) characteristics of vanadium dioxide (VO₂), a highly active tunable terahertz (THz) metasurface resonator patterned VO₂ cross structures is proposed. The simulation results show that the transmission of the proposed structure at low temperature is higher than 0.7 because of the VO₂ in its insulation phase. With the increasing temperature, a strong transmission dip appears at 0.76 THz due to the metal phase of VO₂, which indicates the onset of a new resonant mode. The maximum tunable range of transmission for THz wave is 0.06-0.86. Simultaneously, we also demonstrate that the resonant responses of the heating and cooling processes are quite different and the heating process is more sensitive. The transition temperature is close to room temperature and the device can achieve good modulation effect on both TE and TM waves. The TE and TM mode resonances are selective by altering the cross arms, which is almost impossible for other THz devices. So, the resonator can greatly promote practical applications of THz functional devices such as filters, sensors, modulators, and switches.

Gm-APD arrays lidar has the advantages of long imaging distance, small volume and low power consumption. Because of its unique high resolution three-dimensional range profile, it is expected to solve the problems of UAV safe flight, autonomous obstacle avoidance and so on. In this paper, according to the dynamic imaging requirements of UAV lidar, a joint image stabilization control algorithm of adaptive Kalman filter and PID is proposed to suppress the disturbance of UAV platform to lidar system and make the laser beam point to the target stably. The vibration test experiment of Gm- APD lidar system is made. Under the condition of horizontal amplitude 5mm and frequency 15Hz sine wave disturbance, the gyro drift is less than 1.7 °/ s, and the target drift is no more than 4 pixels. It is proved that Gm-APD lidar can be applied in the field of UAV safe flight.

At present, the research on hollow light mostly focuses on the near-infrared invisible light band, however, with the development of space light field and nonlinear optics, the nonlinear frequency conversion process of hollow light is also paid attention to, but most of them are focused on the theoretical level of frequency transformation, and few hollow light frequency conversion with realizability and applicability is available. In this paper, the visible hollow light conversion from 1064 nm to 532 nm is realized by using the conical refraction of biaxial crystal and a novel technical scheme based on the plane concave cavity structure and combining with nonlinear frequency conversion. The gain medium Nd:YVO4 is pumped by 808 nm LD, and LBO I phase matching is used for intra-cavity frequency doubling. When the pump power is 10.49W, 532nm hollow light output is 0.067W; finally, when the pump power is 23.09W, the maximum output power of 532nm hollow light is 1.146W, and the maximum light-to-light conversion efficiency of 1064nm-532nm is 27.4%. During the experiment, the cone refraction led to the formation of the 532nm hollow spot was observed, and the output performance of the 532nm hollow light was analyzed. The experimental scheme in this paper has the advantages of simple structure, low maladjustment sensitivity and high mechanical stability, which is conducive to frequency conversion in resonant cavity. It provides a novel and effective technical means for obtaining hollow light in visible light band, and broadens the application of hollow light in quantum computing and communication.

The microstructure, mechanical and electrochemical properties of dissimilar materials welded joints, which conducted by laser dual-beam, composed of dual-phase stainless steel DP 780 and 5083 aluminum alloy has been investigated. Microstructures of the joints were studied using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Properties of the joints were evaluated by performing micro-hardness tests and electrochemical tests. The results show that the microstructure of weld is consists of three parts, the zone of coarse-grained martensite, the lath martensite zone and the fine-grain martensite zone from bottom to top. The inter-metallic compound FeAl2 is distributed on the edge of the weld on steel-side, while the inter-metallic compound Al3Fe exists as discrete island on aluminum-side. After remelting and recrystallizing, the heat-affected zone micro-hardness of dual-phase stainless steel is harder than that of base material and the weld, while the area with highest micro-hardness of aluminum is the weld. The electrochemical characterization is acquired by the scanning vibrating electrode technique (SVET), the peaks of anodic current density are near to fusion zone, but the peaks of cathode current density are distribute in weld and HAZ on the SVET maps. The corrosion resistance of the aluminum alloy base material and the lower part of weld is better than other areas after prolonged corrosion.

At present, there are some problems in the digital scanning imaging methods of multimode optical fiber, such as poor quality of focused spot, large amount of calculation and long time in the forming process of focused spot. In order to solve the above problems, a parallel phase compensation method based on liquid crystal spatial light modulator (LCSLM) is proposed to achieve fast point-focused scanning of multimode fiber (MMF). The parallel algorithm, which includes the two processes of collecting online specklegrams and calculating offline phase masks, calculates the compensation phase masks so that the phase masks can be loaded on SLM in turn to generate a series of focused spots at different predefined positions, which greatly saves time. Experiments demonstrate that this method doesn’t need many iterations, and the phase compensation mask used for focusing spot at a predefined position on the MMF output facet can be obtained by using a two-step phase-shifting technique, and a series of phase compensation masks can be quickly obtained by using the parallel method, so as to form focused spots at different predefined positions. Experimentally, we obtained 100 focused spots, the average focused efficiency was 30.12%, the average focused diameter was 2.3382μμm, and the quality of the focused spot was improved compared with previous reports.

The image distortions caused by the inherent mode dispersion and coupling of the multimode fiber (MMF) lead its output light field to be scattered and prevent it from applicating in endoscopy. Although various wavefront shaping methods have been proposed to overcome these image distortions and form the focused spots through the MMF, they a re usually time-consuming due to the multiple iterations and tedious calculation. In this paper, we present a binary amplitude-only modulation parallel coordinate algorithm for focusing and scanning light through a multimode fiber (MMF) based on the digital micro-mirror device (DMD) in a reference-free multimode fiber imaging system. In principle, our algorithm is capable of efficiently calculating the masks to be added to DMD for yielding a series of tightly focused spots; and for the same number of modulation sub-regions, our method is more than M (the number of focused spots) times faster than the amplitude iterative optimization algorithm. In the experiment, efficient light focusing and scanning at the distal end of the MMF without the iteration process are demonstrated. Furthermore, we demonstrate that the proposed method can also be extended to focus and scan light at multiple planes along the axial direction by just modifying the input wavefront accordingly. We predict the high-speed focusing method through the MMF might have the potential application for fast spot-scanning imaging.

We present a compact pulse width stretched nanosecond pulse Nd:YAG green laser based on a multi-pass cavity (MPC). The pulse widths were stretched from adjustable regime 110-260 ns to 460-600 ns by changing from single to multi-pass operation at the pulse repetition rate of 10 kHz. At a typical pulse width of 460 ns, an average output power of 6.5 W was successfully achieved. The 532 nm output power and pulse width versus the 1064 nm pump power were simulated. The calculated results were well consistent with experimental data. Such nanosecond pulse lasers are interesting for both industrial and scientific applications, for example laser damage experiment for some new materials.

We present a study of the saturated absorption spectroscopy on cesium D2 line (6S_1/2F = 4 → 6P_3/2) induced by a multiwavelength laser and its specificity. Compared with the traditional saturated absorption spectroscopy induced by single-frequency laser, the major difference is that the peak amplitude can reach more than half of the Doppler background, and the second is that the number of the saturated absorption peaks is increased and not in the same direction, the line shape and amplitude of the spectrum change with the magnetic field. The frequency components of the multi-wavelength laser need to be studied in order to further analyze the principle of generation of the spectrum. The cesium atom laser spectroscopy induced by this unique multi-wavelength laser can be used for laser frequency stabilization. The interaction of multi-wavelength laser and atom can increase the utilization rate of the atoms, increase the signal amplitude, help to improve the signal-to-noise ratio, and then improve the frequency stability of lasers. In this experiment, the multiwavelength laser spectroscopy induced by the superposition of saturation effect, absorption effect, optical pumping effect and other physical effect is of great interest in high resolution laser spectroscopy and laser frequency stabilization.

The traditional optical system is composed of multiple optical elements, which will lead to the huge volume and high cost of the optical system, which can not meet the needs of people in terms of performance. The emergence of metasurface has made a great breakthrough in the problems existing in optical systems. In recent years, the metasurface has attracted the attention of researchers because it can breakthrough the limitation of traditional electromagnetic law and realize the modulation of light phase, amplitude and other parameters. In this paper, a kind of multiwavelength confocal plane multifocal metalens in visible wavelengths is designed. The design of metalens is realized by computer generation holography(CGH), and the phase distribution of the metalens is analyzed and calculated. Transmission metalens designed with all-dielectric materials is used and the imaging effect of confocal plane four-focal metalens at different wavelengths of visible wavelengths is analyzed. The surface phase characteristics of the multifocal metalens at different wavelengths were analyzed, and the Pancharatnam-Berry (P-B) phase was used to match the rotational phase of the unit cell structure with the phase of the surface of the metalens, and the confocal plane focusing at multiple wavelengths was successfully realized, with the focusing efficiency reaching 43%.

The nonlinearity and dispersion characteristics of a suspended core fiber with different fluid filling methods and slot-structure-embedded core configuration is simulated and analyzed here. Firstly, four suspended core bismuth fibers were filled with high nonlinear fluid CS₂ fluid by selective filling method. With the selective filling method, due to the mode redistribution effect, the nonlinearity of optical fiber is significantly enhanced, and the dispersion characteristics of optical fiber can be customized more smoothly. Furthermore, an elliptical, nonlinear fluid-filled nanoscale slot structure is embedded into the suspended Bismuth fiber core for mode distribution adjustment and nonlinearity enhancement, while the cladding air holes are partially filled with nonlinear fluid CS₂ for mode field compression and nonlinearity enhancement. Finally, the fiber parameters and the slot size are tailored properly for adjusting the fiber dispersion characteristic. Simulation results show that an ultrahigh nonlinear coefficient that reaching up to 12023W^-1km^-1 and a flattened dispersion with fluctuation less than 22ps/Km/nm at C-band are possible after the optimization.

The study of the interaction between laser and brain tissue has important theoretical and practical significance for brain imaging. A two-dimensional simulation model that studies the propagation of light and heat transfer in brain tissue based on finite element has been developed by using the commercial finite element simulation software COMSOL Multiphysics. In this study, the model consists of three parts of 1) a layer of water on the surface of the brain, 2) brain tissue and 3) short pulsed laser source (the wavelength is 840nm). The laser point source is located in the middle of the layer of water above the brain tissue and irradiates the brain tissue. The propagation of light in brain tissue was simulated by solving the diffusion equation. And the temperature changes of gray matter and blood vessels were achieved by solving the biological heat transfer equation. The simulation results show that the light energy in the brain tissue decreases exponentially with the increase of penetration depth. Since the cerebral blood vessels have a stronger absorption on light compared with the surrounding tissues, the remaining light energy of the blood vessels in the cerebral cortex is ~ 85.8% of the remaining light energy in the surrounding gray matter. In the process of biological heat transfer, due to more light deposition in blood vessels, the temperature of blood vessels is 0.15 K higher than that of gray matter, and the temperature of gray matter hardly changes. This research is helpful to understand the propagation of light in the brain and the interaction between them, and has certain theoretical guiding for the optical imaging of the brain.

In this paper, we present systematically theoretical analysis of the high order mode (HOM) oscillation in large mode area (LMA) fiber oscillator. A detailed numerical laser oscillator model based on LMA fiber has been built. Model analysis shows that the HOM begins oscillating in fiber laser oscillator when the fiber core diameter reaches 25μm. This HOM oscillation could seriously degrade the beam quality of laser output. In order to suppress the HOM oscillation in LMA fiber laser, we put forward two fiber device models which are partially inscribed fiber Bragg gratings (FBGs) and partially doped gain fiber. Simulation proves the inhibition ability against HOM oscillation of these two devices. Our work provides a complete and concise analysis on the mode characteristics in high power fiber laser oscillator.

Laser surface texture and atmospheric plasma spraying technology were used to prepare molybdenum coating on AISI 304 stainless steel substrate. A 3D surface topography instrument was used to extract the groove profile and a quadratic parabola was used to fit the textured groove. Meanwhile, a numerical model was established in which multiple droplets filled a single measured and fitted groove. The solidification, defects, and wall temperature of the coating filled with the groove with droplets were compared with the parabola grooves. The simulation comparison between the fitted groove and measured groove shows that the splat was opening angle increases by 22.95%. The maximum heat flow rate transferred by the groove wall interface decreases by 10.1%. The solidification time trend is the same, whereas the filling splash of droplet fluid is reduced.

Large bridges and industrial equipment may encounter natural disasters (earthquakes, tsunamis, etc.) and man-made effects during their service period. When they are subjected to these external influences, the structure may be deformed or even cracked, and the internal stress of the structure can also cause the occurrence of acoustic emission events. In this work, we report a fiber-optic acoustic emission sensing system using a semiconductor optical amplifier (SOA)-based fiber-ring laser source including a non-tunable fiber Fabry-Perot filter (NTFFPF) to demodulate dynamic signals from fiber Bragg grating (FBG) sensors. The shift in the FBG reflection spectrum caused by external strain is demodulated by the NTFFPF in the ring laser cavity, which ultimately produces an amplified output signal. The proposed system was used to detect the high-frequency acoustic emission signals generated by the piezoelectric buzzer. Experimental results show that this system can demodulate high-frequency acoustic emission signals with a good response and a high signal-to-noise ratio up to 21.6 dB. At the same time, acoustic emission signals generated by an ultrasonic vibrator with a frequency of 40 kHz are detected simultaneously with a FBG sensor and a piezoelectric sensor placed in the middle of a square aluminum plate. The angle-dependent acoustic emission measurement is performed by placing the ultrasonic vibrator at different angles from 0° to 90° in the radial direction of the FBG sensor. The results show that the sensor system can accurately detect the high-frequency acoustic emission signals on the aluminum plate and larger signal amplitude can be obtained when the angle between the ultrasonic vibrator and the FBG sensor axial is in the range of 0-60°. The fiber ring laser sensing system proposed in this paper has application prospects in many aspects, such as acoustic emission source location and ultrasonic detection.

High precision time measurement technology is the basis of many scientific applications. It plays an important role in radar, sonar, laser ranging, particle physics and other advanced scientific fields. Meanwhile, the ability of high-precision time measurement is also an important factor limiting the development of these fields. Time to digital conversion (TDC) is a commonly used time interval measurement method, which is widely used in the above-mentioned advanced areas. This paper presents a new TDC design method based on the delay chain structure, and the measurement accuracy is further improved by the combination of rough and exquisite counting. The theoretical basis of the TDC is described. The calculation formula of the total time to be measured is given. Then, by combining the start and the stop signal reasonably, the time of gate signal is within an acceptable range, which reduces the instability of the signal and the number of input signal sources. The designed TDC achieves excellent delay uniformity and stability through the reasonable layout and routing of Carry4 delay chain module in FPGA. In addition, in the design of latch unit, a two-stage latch unit is designed according to the mean time between failures (MTBF) theorem, which ensures the consistency of delay and the correctness of timing, avoids the generation of metastable state, and improves the timing accuracy. Finally, in order to verify the performance of the proposed TDC design scheme, reasonable post simulation and board level verification are conducted under different clock frequencies. The verification results show that the maximum mean error of TDC is 3.99ps and the minimum is 2.82ps.

High-spectral-resolution-lidar (HSRL) can realize the retrieval with high accuracy of aerosol optical properties without the assumption of atmospheric conditions, in which the key technique is to construct a spectral discriminator for separating aerosol Mie scattering signals and molecular Rayleigh scattering signals. Both Mach-Zehnder interferometer and Fabry-Perot interferometer have been developed and applied in the HSRLs, as they can provide the fine spectral structure in signal separation. In this paper, we make the performance comparison between the Mach-Zehnder interferometer and Fabry-Perot interferometer from the points of view of single-frequency HSRL and multi-longitudinalmode HSRL. The simulation results show that the design parameters of Mach-Zehnder interferometer and Fabry-Perot interferometer are different as the spectral discriminator in the design of HSRL to separate aerosol Mie scattering signals and molecular Rayleigh scattering signals. Meanwhile, to match the free spectral range of optical interferometer with the laser longitudinal mode interval in the multi-longitudinal-mode HSRL, Mach-Zehnder interferometer is a better choice compared with Fabry-Perot interferometer, although both can provide the periodic spectral transmittance functions.

Employing MoS2 as a saturable absorber, We firstly demonstrated a passively Q-switched mode-locked (QML) Tm,Ho:CaYAlO4 laser. A tunable wavelength Ti:sapphire laser is used as the pumping source, and the continuous operation threshold of the Tm,Ho:CaYAlO4 laser is as low as 191 mw. When the absorption pumping power reached 1210 mW, the Tm,Ho:CaYAlO4 laser will entered a stable Q-switched mode-locked operation state, further increasing the pump power to 2.6w, the maximum output power of the laser is 216 mW, which corresponds a slope efficiency of 9.32% at the central wavelength of 2089 nm. In this case, the modulation depth is close to 100%. the experiments show the potential application of MoS2 in mode-locked operation at 2 micron band.

At present, the finite element method is often used to analyze the thermal effect of laser medium in solid-state lasers. By calculating the temperature field distribution, further research on thermal lens, thermally induced depolarization and other thermal effects can be carried out. In the thermal analysis model of laser medium based on heat conduction equation, the definition and selection of characteristic parameters have an important influence on the calculation results. Due to the difference and diversity in the definition and selection of characteristic parameters, this paper focuses on the definition of heat source in the model, gives the expression of heat source under different spatial distribution of pump light, analyzes definition of conversion coefficient of pump energy into heat deposition in crystal, and limits the range in boundary conditions. Taking the thin disk laser medium as an example, the influence of the model parameters on the calculation results is analyzed and the accuracy of the model is verified compared with the test results.

The scheme of N×1 multi-beam combination with single gradient-index lens (GRIN) was proposed and the coupling model was simulated with ZEMAX software. One end of the GRIN was fabricated to polyhedral convex cone with N surfaces and taken as output port, and multiple beams of light converge after passing through this plane. The influences of length of GRIN lens, angle of inclined surface and distance between of fiber end and GRIN lens on coupling efficiency were analyzed. The results show that this kind of combiner has very high efficiency for multi-beam coupling, and the coupling efficiency of N×1 fiber pump combiner reaches 97.64%. When the length of GRIN lens was between 7.36mm and 7.96mm, the coupling efficiency of the beam combiner can be maintained above 95%. When the slope angle φ is 40.995︒ <φ< 43.095︒, the coupling efficiency of the combined beams were greater than 97%.

The directional polarimetric camera (DPC) is a satellite sensor with the aim to observe the polarization and directionality of the earth’s reflectance, developed by Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. The analog front-end signal processor is the key to realize the conversion from the analog input signal of the front-end CCD image to the digital processing signal of the back-end. Its performance directly determines the image quality of the instrument. In this paper, the single particle locking effect of the CCD analog front-end signal processor of the Directional Polarization camera after being irradiated by high-energy particles is studied, the test scheme and the test system composition of the single particle locking effect are introduced, and the dynamic working current of the device and the change rule of the dynamic output digital code signal and other parameters are analyzed. The experimental results show that when the LET threshold of high-energy particles is greater than 13.4 MeV•cm²•mg-1, the dynamic working current of the analog front-end signal processor increases significantly, the dynamic output digital code cannot change with the change of the input signal, and single particle locking occurs in the chip. These research results provide significant reference data for in-depth study of single particle locking protection of analog front-end signal processor in space application environment.

The effect of LiNbO₃ crystals temperature on the formation of periodic surface structures induced by femtosecond laser pulse with duration of 120 fs and wavelength of 800 nm was investigated theoretically and experimentally. Firstly, experimental results show that surface periodic ripples tend to form on high temperature LiNbO₃ crystals. Then, an improved rate equation including Drude model was proposed to analysis the dynamic of free carrier density and optical properties. The theoretical calculation results show that, the LiNbO₃ crystal at high temperature is more likely to excite high-density carriers under femtosecond laser irradiation, which benefits from the change of carrier excitation mechanism. Temperature-controlled femtosecond laser micro-/nano- processing will become a new effective method for fabrication wide-gap transparent dielectric materials.

A simple and novel heterodyne interferometer scheme with spatially separated input beams was proposed to minimize the periodic non-linearity. A polarizing beam displacer (PBD) was designed to split the input beams to parallel outputs with orthogonal polarization or combine them which provided symmetric structure for the interferometer. The novel optical setup suppressed the non-linearity caused by the frequency-mixing. Potential use in the measurement of micro displacement or vibration of the new interferometer will be demonstrated.

The earth’s gravity field reflects the spatial distribution, movement and change of the earth’s mass. High precision gravity field is one of the main tasks of geodesy. The feasibility research of future China's satellite gravity mission(CSGM) were carried out, the satellite to satellite tracking in the low low mode was selected in the future satellite gravity measure. A precise laser ranging system is a key component to CSGM. In this paper, a laboratory breadboard heterodyne laser interferometer was built, experimental result demonstrated that laser ranging performance in laboratory distances was 1 nm@0.01Hz and below1nm@1Hz in translation measurement, and can meet the demand of CSGM mission.

We experimentally demonstrate a diode-pumped passively Q-switched Yb:Lu₃Al₅O₁₂ ceramic laser operating at 1031 nm or 1047 nm based on a semiconductor saturable absorber mirror (SESAM) and an output coupler (OC) of different transmission (T). For an OC of T=7.5% and an absorbed power of 9.25 W, the laser operates at 1031 nm with a maximum average output power of 1.47 W, the corresponding optical-to-optical efficiency of 15.85%, and the slope efficiency of 19.04%, respectively. The pulse width and the pulse repetition frequency (PRF) are about 1.23 μs and 245.04 kHz, respectively. For an OC of T=1.6% and an absorbed power of 9.25 W, the laser operates at 1047 nm with a maximum average output power of 0.85 W, the corresponding optical-to-optical efficiency of 9.21%, and the slope efficiency of 8.43%, respectively. The pulse width and the PRF are 1.49 μs and 112.63 kHz, respectively.

In order to quantitatively analyze the high-precision traveling wave tube (TWT) electron gun anode welding micro deformation during the process of pulse laser beam welding (LBW), and meeting the terahertz electron vacuum device concentricity design requirements, the welding process of actual anode(Mo1) and anode positioning part(4J33) were studied. Eight LBW spots were pre positioned in the circumferential direction, then four symmetrically and uniformly sections were welded for reinforcement. Detection and compensation were carried out during and after welding. After considering the fit clearance, geometric measurement uncertainty, spot positioning accuracy and welding efficiency, a set of high-precision LBW deformation detection and compensation system was developed . The results showed that the concentricity of anode hole and gun end cover was better than 10μm; the eccentricity caused by the first two LBW joints was larger, about 5-10 μm; the deformation caused by the third spot was 2-4 μm, and that of the next 4-8 spots was less than 2 μm. After 8 spots were pre positioned, the deformation caused by the following four sections was less than 2 μm. After high-temperature baking at 550 °C for 72 hours, the deformation was less than 3 μm. The tensile strength of the weld could reach to 13.2N, which could ensure the anode parts which only weigh 2g could withstand enough mechanical acceleration impact. Compared with the one-time welding four sections, the concentricity was optimized by 60%, and the deformation of high-temperature baking was optimized by 50%.

We demonstrated the noise-like pulse (NLP) generation in an erbium (Er) doped fiber laser at 1560 nm based on 2HMoTe₂. The saturable absorber (SA) was a film mixed by 2H-MoTe2 and polyvinyl alcohol (PVA). The moudulation depth of the SA was 21.4% and the saturable optical intensity was 10.3 GW/cm². When the 2H-MoTe₂ film was inserted into the cavity, the stable NLPs with 3-dB spectral bandwidth of 2.44 nm and the repetition rate of 33.7 MHz were obtained. The autocorrelation (AC) trace of NLP has a 1.14 ps spike and a 71 ps pedestal. When the pump power was 600 mW, the maximum output power was 26.09 mW. To the best of our knowledge, this is the first time to realize the generation of NLP mode locking in an Er-doped fiber laser based on 2H-MoTe₂-PVA film.

Repetition frequency laser used in precision strike system can transmit a series of laser pulse trains. The repetition frequency pulse transmitted by common repetition frequency laser device does not meet the high precision measurement and target indication requirements due to the existing of “relaxation oscillation” effect. Thus, a Laser Q modulation technology must be used for the laser pulse shaping. Q switch is formed by applying high voltage to the electro-optical crystal cut at a certain angle through electro-optical Q modulation technology. At present, the Q switch of the high repetition frequency laser system is easy to be damaged under the state of high repetition frequency. “Piezoelectric bell response” exists with the over-high voltage and that will affect the repetition frequency accuracy. Aiming at LGS crystal applied in the repetition frequency laser equipment, we carry out an experimental research in order to solve the technical problems, such as optical rotation effect, frequency stabilization driver, and develop LGS Q switch, driver module and repetition frequency laser system. The experiment results show that the Q switch applied in the high repetition frequency laser system is capable to produce the anti-damage and high repetition frequency effect.

When the laser terminal guidance system is working, the irradiator emits re-frequency laser pulses that precisely control the light time, so as to provide the seeker with an accurately identified signal source, while avoiding enemy laser interference. The laser irradiator uses electro-optic Q-switched technology to obtain pulses with high pulse energy and precise time control. An experimental device for testing the output pulse time of the electro-optic Q-switched laser was designed. The experimental analysis the action process of the electro-optic effect of the KD*P crystal. The effect of the delay time of Q-switched on the output energy and pulse width of the laser was tested and discussed. The results show that the setting of the optimal delay time can increase the output energy of the laser, which has a positive significance for controlling the light time of the irradiator of laser terminal guidance system.

We studied ring width of the Laguerre–Gaussian vortex beams, which is the outer radius minus the inner radius. Combining theoretical analysis and numerical simulation, the approximate expression of the ring width is obtained. From the expression, we find that the ring width is independent of the topological charge, and increases with the increase of the waist radius at z=0 of the host Gaussian beam and the propagation distance. This research has good application prospects in optical tweezers technology and OAM optical communication.

Qualitative and quantitative analysis of boron (B) in ceramics is of great significance to the preparation of ceramics. According to the relevant content of "Ceramic Technology", boron (B) can effectively reduce the melting temperature range of the glaze, and on the other hand, it can also reduce the surface tension of the glaze. Laser-induced breakdown spectroscopy (LIBS) technique is expected to be one of the most preferred techniques in archaeology since it does not damage the structural and chemical component of cultural relics. It almost could achieve all elements in the periodic table. In this work, LIBS is used for analyzing boron (B) element in ceramics. By utilizing MobiLIBS system produced by the French company IVEA, this machine is equipped with automatic calibration curve module. We could build up the standard curve and uses the calibration curve to detect the boron element quantitatively. These results can provide the reference value for the restorers.

Vortex light is used widely in quantum entanglement, optical communication, high-resolution imaging, optical detection, optical manipulation, and many other fields. In practical applications, stable and adjustable vortex light sources are needed for applying the orthogonal characteristics of vortex light with different orders. We demonstrate a tunable vortex laser based on an intra-cavity aspheric lens. By adjusting the position of the lens and the output coupler, several orders of vortex beams have been obtained at the same pump power. The spatial matching between the pump light and the modes in the cavity determines the preferentially oscillatory transverse mode. Furthermore, we compare the mode selection mechanisms in previous researches on vortex lasers, that is, the mode change caused by the thermal lens effect, and the dual feedback mechanism of the spherical lens with appreciable spherical aberration, and discuss the intrinsic similarity of these methods.

Filtered back-projection (FBP) is a classic imaging method in photoacoustic tomography in both frequency and time domain. However, the FBP algorithm itself will produce large artifacts, especially under extremely low signal-to-noise ratio which makes the acquisition, analysis and processing of photoacoustic signals (PASs) more difficult. Traditional filtering methods may cause modal aliasing of the filtered signal, loss of effective information, and the introduction of new interference in the process of using de-noising algorithms, which increases the difficulty of signal extraction, so select the appropriate filtering method has become an important part of this imaging mode. In addition, many algorithms tend to ignore the phase delay caused by filtering, which has a great impact on the accuracy of cross-correlation processing and distance inversion. According to the physical and mathematical principles of each step of the algorithm, this paper introduces the Empirical Mode Decomposition (EMD) algorithm to decompose the collected PASs, and puts forward the non-phase delay sifting proposal without introducing additional phase delay. A new strategy aimed at linear frequency modulation light sources under the framework of FBP algorithm. By keeping the center frequency of the ultrasonic transducer and the modulation signal unchanged, the validity of the algorithm is verified through experiments, which improved the reconstruction quality of the image. The new EMD strategy can effectively extract low signal-to-noise ratio of PASs and complete the photoacoustic tomographic reconstruction in FBP mode with higher quality.

As a kind of metal material with low density and high strength, Al alloy plays an important role in transportation, electric power and energy fields. Enhancing its corrosion resistance is of great significance to improving the application value. In this paper, the groove array micro-structure was fabricated on the surface of 5083 Al alloy by laser, and the surface free energy was further reduced to obtain a superhydrophobic Al alloy surface. The controllable laser fabrication process of the micro-structure and the corrosion resistance of the superhydrophobic surface were studied. Under the combined impact of laser ablation and in situ deposition, the groove depth first increases and then decreases, and the uniformity gets worse, as the number of scanning increases. The groove structure and hump structure can be obtained successively with the gradual increase of scanning interval. EDS results show that the materials sputtering to both sides of the groove had a most intense oxidation, and the ratio of oxygen element increased by 11.2%. The largest contact angle of the fabricated superhydrophobic surface can be 162°, which shows little adhesion. The electrochemical corrosion test results show that the current density has dropped by two orders of magnitude, and the corrosion inhibition rate is 98.9%, showing good corrosion resistance.

Phase retarders (PRs) are space-variant waveplate elements which are usually utilized to generate radial and azimuthal polarization and find the polarization axis. So far, there have been mainly two types of phase retarders: half-wave phase retarder and quarter-wave phase retarder. Their principles are similar while functions are different. Some researchers may feel confused to make a choice between them. The present work aims to provide an engineering reference for choosing phase retarders with half-wave and quarter-wave retardation in practical applications. We make a comparison of the two types of PRs in both principles and applications to show their properties clearly.

Generating reference on the back focal plane (BFP) of objective lens has been an excellent solution for numerical aperture (NA) measurement. Although the effectiveness of the concept of BFP reference has been verified theoretically and experimentally, there has been no practical apertometer based on this concept. The main challenge is how to realize the assembly of the system. In this work, we present the instrument assembly of the apertometer based on BFP reference, which mainly has three requirements. After that, we specifically investigate the influence of the location of the coverslip on BFP image and NA value by using ZEMAX. This work promotes the instrumentation of BFP-typed apertometer significantly.

In this article, the scattering transmission of laser beam in the dust haze with low visibility is studied, which provides a reference for the laser beam experiment under the condition of air conditions with dust haze. According to the gamma distribution of particle size of sand dust particles, the attenuation of laser beam at different wavelengths of sand dust particles with different water content is studied by using The Discrete Dipole Approximation (DDA) and Rayleigh approximation equivalent method of equal volume sphere. The results show that the greater the water content, the greater the attenuation of laser beam; the comparative calculation of dust particles and haze main components at different wavelengths. The results show that the absorption factor of dust particles is larger than that of other haze components except carbon sol, while the scattering factor is slightly smaller than that of haze components. Considering the different proportions of dust haze components and visibility, Monte Carlo method is used to study the transmission transmittance and reflectivity of dust particles in dust haze. The attenuation characteristics of the laser beam caused by the dust haze particles in the state of coated water layer and the dust haze particles in waterless coating condition layer are calculated. Through the research and calculation of this paper, we can clearly and intuitively see the attenuation characteristics of laser beam caused by dust haze. Therefore, it is very necessary to study the attenuation of laser beam caused by dust haze.

As a new laser welding, laser oscillating welding has the advantages of better gap adaptability, less porosity and higher strength compared with conventional laser welding. Moreover, due to beam oscillation, the variation of molten pool flow contributes to the formations of porosity and cracks, welding stability, microstructure and properties. Thus to the poor welding stability and porosity in welding of TB8 titanium alloy, laser oscillating welding is applied in the way of transverse and longitude pattern. The results show that the traditional transverse oscillating welding still has a small amount of spatter, but spatter can be avoided by adjusting the energy flux of weld edge in transverse or longitude oscillating welding. Oscillation beam is good for flow of molten pool because of the excursion effect of laser beam stirring on fluid. At the end of welding pool, vortexes that are opposite to each other and distributed on both sides of welding pool plays an important role on grain refining. Oscillation beam also suppresses porosity with low frequency transverse oscillation (80Hz) or high frequency longitude oscillation (150Hz).

A nested resonant ring structure composed of double split square ring based on frequency selective surface (FSS) is proposed to realize ultra-wide bandstop filter. To begin with, the structure is supposed by a single layer, adding the number of resonant rings and metal-dielectric layers to adjust frequency points and bandwidth. Through numerical simulation, we found that the location of the frequency points depended on horizontal nested rings and the transmission coefficient improved by multiply layers. In order to enhance the flatness of stopband, the depth and size of slots are calculated, showing 1.72THz stop-band from 1.27THz to 2.99THz. The result demonstrates that the range of bandwidth achieve 1.5THz with transmission depth up to 20dB, and flat bandstop as well as out-band rejection are well-performed compared with traditional slot structure.

The brain is the main part of the central nervous system, which consists of the cerebrum, cerebellum, brainstem and other structures. There are many nerve centers in the brain. Therefore, the brain has the most complex structure and the most perfect function. It is necessary to study the brain. Since the skull has a strong scattering effect on light, it is difficult for photons to reach deeper through the skull. In order to realize the imaging of brain, methods such as optical clearing, thin skull or are commonly used at present. In order to explore the influence of skull on photon transmission, this paper adopts Monte Carlo method, uses MCML simulation program, and studies the transmission of photon in brain tissue under the condition of with skull and without skull respectively by setting appropriate parameters. The influence of skull on photon is analyzed by photon trajectory, optical absorption density and fluence rate. It was found that, from the analysis of the motion trajectory of the photon, compared with that of the photon with skull, the motion ability of the photon without skull was enhanced in the radial direction and depth direction, and the distribution of the photon number increased along the longitudinal and radial direction. The optical absorption distribution and fluence rate distribution obtained without skull are fusiform and fatter than those obtained with skull, while the light distribution obtained with skull is smoother. The optical absorption distribution and fluence rate distribution without skull were more widely distributed in radial direction, with a range of 0.46 cm, 4.6 times that with skull. The depth of transmission was deeper, about 2.1 cm, while the depth of transmission was only 1.9 cm with skull. This research will provide reference and theoretical guidance for brain optical imaging.

The propagation characteristics of high-average-power tube laser from a hemisphere turret on an airborne platform is studied theoretically. Based on the theoretical model for the four-dimensional propagation model of tube laser beams, the influences of aero-optical effect, atmosphere turbulence and thermal blooming effect on the propagation characteristics of the tube laser are simulated and analyzed. The results show that the aero-optical effect contributes dominantly to the degradation of the tube laser under the conditions of high speed and high altitude and further brings about severe beam spreading and centroid shift. Owing to the low air density at high altitude and high heat exchange efficiency at high speed, the atmosphere turbulence and thermal blooming effect are extremely weak. This result implies that the areo-optical effect should be paid more attention for the realization of long-distance propagation of tube lasers on high altitude flights.

We demonstrated a broadband Nd: glass laser amplifier with controllable gain bandwidth in experiment, which is expected to be used in high energy low coherent laser amplifier system. A birefringent crystal for spectral filtering in regenerative laser amplifier was designed and proposed, and the parameters of birefringent crystal was optimized in simulation. The birefringent crystals with different bandwidths were designed by this method. The laser amplification output with different gain bandwidths of 6~10 nm was realized, which was also verified by experiments. This broadband Nd: glass laser amplifier is expected to be used in large scale broadband laser amplification system, and provides important technical support for a new type of broadband tunable nanosecond high-power laser facility.

The rapid development of endoscopic photoacoustic imaging technology with the combination of endoscopic imaging and photoacoustic imaging has a great demand in clinical practice. Because the volume of the internal snooping head in the current study is limited by the volume of the ultrasonic transducer, the ultrasonic transducer with high sensitivity and small volume needs to be further explored and studied. We use a two-photon 3D lithography machine to process the well designed Fabry Perot(FP) cavity on the end of the double-clad fiber as the sensor probe, the two road light coupling into the double cladding fiber respectively, in which fiber core transmission of light as the excitation light all the way, in the first a pack of transmission between the layers of the reference light all the way, the use of the "spontaneous since" mode for nondestructive ultrasonic testing. Based on the design of F-P microcavity as a detection unit, a new all-optical detection system for endoscopic photoacoustic imaging is realized. It has great potential in clinical application research.

Carbon fiber reinforced plastic (CFRP) is a key material for lightweighting in aerospace, automotive and other fields. It has the characteristics of anisotropy, high strength, and high hardness. In the processing of CFRP, the use of traditional machining has disadvantages such as severe tool wear, obvious delamination, micro-cracks, and changes in material properties during processing. Laser processing for CFRP provides a non-contact and wear-free processing method. However, it will cause the epoxy resin to generate the heat load, leading to the heat affected zone (HAZ) during the processing. The HAZ is the important indicator of the cutting quality and directly affects the material mechanical properties. Femtosecond pulsed laser with its "cold processing" advantage can greatly reduce the laser's thermal damage to materials and reduce the material's HAZ. In this paper, the cutting process of CFRP by ultraviolet femtosecond pulsed laser is studied. A UV femtosecond laser with a maximum average power of 15W was used to conduct cutting experiments on 2mm thick CFRP. Using the experimental method of single factor analysis, the effect of laser average power, repetition frequency, scanning speed, scanning times, scanning track spacing on cutting quality was studied. The results show that when the average power is 13W, the repetition frequency is 500kHz, the scanning speed is 9m/s and other appropriate process parameters, the width of the HAZ is about 25um.

Based on optofluidics and the principle of total internal reflection, a 1×2 few-mode waveguide optical switch is proposed. The proposed few-mode optical switch has small volume, low insertion loss and simple structure. It can be directly connected with a few-mode fiber to reduce the coupling loss. The mode field distribution and transmission characteristics of the two-mode switch (LP01 and LP11) are discussed. After the optical switch structure parameters are optimized, the minimum insertion losses are respectively 0.54 dB and 0.42 dB at two output ports, and the crosstalks are respectively -20.4 dB and -48.0 dB. The transmission characteristics of the four-mode switch (LP01, LP11, LP02 and LP21) are also analyzed. The insertion losses of four-mode optical switch are less than 0.34 dB, and the crosstalks are less than -23.19 dB. Our work provides a new method for seeking high-performance, miniaturized and integrated fewmode optical switches.

This paper reports the experimental research of supercontinuum (SC) generation in a 1030 nm fiber amplifier for the first time. The ytterbium-doped fiber amplifier designed based on the 1030 nm seed laser is used to pump the passive fiber to obtain the SC output. In this paper, the effects of the pulse width and repetition frequency of the seed laser on the SC output power and spectral characteristics, and the influence of the passive fiber length on the SC output characteristics are experimentally researched. The experimental results show that under the condition that the peak power of the amplifier is basically the same, the output power of the SC spectrum is basically unchanged by increasing the pulse width and reducing the repetition frequency of the seed laser, but the ability of the spectrum to broaden toward the edge of the long-wavelength becomes stronger and the long-wavelength band in the spectrum has a higher proportion of the optical power. Under the conditions that pulse width and repetition frequency of the seed laser and the output power of the amplifier are constant, the longer the length of the passive fiber, the stronger the ability of the spectrum to broaden toward the edge of the long-wavelength. However, a too long passive fiber will transfer the excessive power to more than 2μm, which is not conducive to the further increase of the output power due to the introduction of more losses.

Spectral beam combining (SBC) is a regular approach of utilizing semiconductor laser arrays, as it can greatly improve output power and maintain the beam quality. External cavity spectral beam combining based on the six elements quantum cascade laser (QCL) array, with a 10μm ridge width and a pitch of 60μm, was realized. The divergence of the light from output coupler is 0.6mrad and 3.8mrad, for fast axis (perpendicular to the array) and slow axis (parallel to the array), respectively. Under a condition of 10kHz repetition frequency and 1μ s pulse width, the array’s peak power is 2.9W without SBC system. The peak power of 0.9W and 1.34W is achieved when utilizing a output coupler based on Ge without anti-reflection (AR) coating or with AR coating, respectively. Corresponding the beam combining efficiency is 31.0% and 46.2%, respectively. We recorded the cross-locking phenomenon when tuning array by rotating the external cavity output coupler angle, as a relatively close element spacing results in mutual locking of adjacent elements. The system could tuning over 99cm^-1 (1904cm^-1--2003cm^-1).

A compact optically pumped vertical-external-cavity surface-emitting-semiconductor laser (OP-VECSEL) at 1178 nm for μs pulsed operation is reported for the first time. The laser employed a V-shaped resonator with the cavity length of 300 mm, pumped by a 200 μs pulse 808 nm laser operating at a repetition rate of 1 kHz. The maximum output power about 0.78W was obtained at pump power of ~5.9 W. The corresponding conversion efficiency from absorbed pump power to 1178 nm was 13.2%. Further, by the intra-cavity frequency doubling of the VECSEL, a μs pulsed 589 nm laser has broad prospects in the application of sodium guide stars.

High-power InGaSb/AlGaAsSb quantum well separate confinement structure lasers are fabricated. Threshold currents of the sample lasers range from 150mA to 270mA, turn-on voltages range from 0.51V to 0.59V. Stable high power and high efficiency operation is achieved, output powers at 3A range from 978mW to 1050mW, the power conversion efficiencies at 3A range from 21.3% to 22.9%, slop efficiencies range from 0.35W/A to 0.38W/A. The capability of fabricating reliable high-power 2μm GaSb-based laser is confirmed. The current of one sample was ramped up to 6A, the emitter exhibits a high performance with peak output power of 1320mW at 5.4A，maximum power conversion efficiency of 27.5% and slop efficiency of 0.34W/A.

The satellite laser altimeter requires high-precision on-orbit geometric calibration to ensure the accuracy of the laser altimeter data. However, the calibration method based on undulating terrain may have multiple solutions under complex terrain, which means that the calibration parameters may converge to the local optimal solution. In order to solve the problem, a satellite laser altimeter pointing and ranging calibration algorithm based on simulated annealing is proposed, which can reduce the possibility of the calibration parameters to converge to the local optimal solution. In 10 sets of comparative experiments, there are 2 sets of result converging to the local optimal solution using algorithm based on Monte-Carlo simulation, while all sets of result converge to the global optimal solution using algorithm based on simulated annealing. After calibration with the proposed algorithm, the average of elevation error decreased from about 9.1m to within 3m, and standard error decreased from about 1m to about 0.5m. The results show that the calibration algorithm based on simulated annealing can effectively prevent the calibration parameters from converging to the local optimal solution, and can effectively improve the accuracy of laser altimeter data.

As the most popular semiconductor material, silicon carbide has received extensive attention from the industry. In this study, a UV laser with a center wavelength of 343𝑛𝑚, a pulse width of 500𝑓𝑠, a maximum average power of 12𝑊, and an adjustable repetition rate of 400~500𝑘𝐻𝑧 was used to conduct cutting process experiments on 4H-SiC. A 3D color scanning electron microscope (SEM) and a metallographic microscope are used to observe the cutting section and surface. The effects of laser repetition rate, laser average power, scanning speed and other process parameters on the cutting quality were studied. The ablation threshold of the silicon carbide was obtained to be 44.62𝑚𝐽/𝑐𝑚2. The best cutting result we made is at the condition with the laser average power 2𝑊, the scanning speed 150mm/s, the repetition rate 500kHz, and the scanning number 22 𝑡𝑖𝑚𝑒𝑠. Using a metallurgical microscope, it was observed that the cutting surface was neat, without chipping or cracking, and the thermal effect was negligible. The 3D color SEM image are shown that the taper of the slit was 5.75° and the roughness of the cut section was less than 2.69μm.

The ability to remove the black crust from stone substrates has great research significance in Cultural Heritage. Laser cleaning has achieved good cleaning results in stone heritage in recent years. However, the extremely high light absorption and low melting point of biotite in the mineral composition of granite can easily cause damage during the cleaning process. In this study, the thermal stress generated by the laser action in the black crust was used to peel off the black crust. And a model of three-dimensional thermal stress was established under the movement of the laser heat source based on the experimental phenomenon of warping to peel off the black crust. The model agrees with the experimental results that the black crust is peeled off by tensile and shear stresses.

In visible light communications, hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) is an attractive scheme with high power efficiency and high spectral efficiency. This paper compares the implementation complexity and receiving performance of three HACO-OFDM systems. The comparison results show that the implementation complexity of the HACO-OFDM system that eliminate noise through time domain signal processing(TD-HACO-OFDM) and the interference-free hybrid OOFDM(IF-HOOFDM) system is lower. In terms of receiving performance, IF-HOOFDM system has the best bit error rate (BER) performance on the asymmetrically clipped optical OFDM(ACO-OFDM) branch; When the clipping noise estimation is inaccurate, the BER performance of the TD-HACO-OFDM system on the pulse-amplitude-modulated discrete multitone (PAM-DMT) branch is significantly better than the other two systems, the BER performance of the IF-HOOFDM system on the PAM-DMT branch is better than the conventional HACO-OFDM system based on frequency domain processing(FD-HACO-OFDM).

There are various scattering media in nature. How to achieve rapid imaging through scattering media is an important issue in the field of optical imaging. In order to improve the speed of scattering imaging, this paper adopts a new type of residual neural network called XRNet, which mainly uses the advantages of easy optimization of residual neural network(ResNet) and combines U-Net structure and depthwise separable convolution instead of standard convolution to reduce the complexity of training model, thereby improving recovery speed of imaging. The image restoration speed and imaging quality under this model are simulated and calculated, and the convolutional neural network (CNN), ResNet and XRNet are analyzed by Pearson Correlation Coefficient (PCC), Peak Signal to Noise Ratio (PSNR) and Structural Similarity Index Measure (SSIM). The results show that XRNet has the fastest imaging speed when the imaging quality is basically unchanged. Finally, the imaging quality and recovery speed of XRNet at different depths are evaluated, and the best values are obtained at a depth of 31 layers.

High-brightness tapered diode lasers generally work in the fundamental transverse mode. Still, under the condition of high power, the beam quality is often deteriorated by the influence of higher-order modes. At the junction of the device ridge waveguide and the conical amplifier, there is a great difference in the refractive index step △n, which is the key area for producing high-order modes and influencing each other. In this paper, the physical process of high-order side mode excitation in a large optical cavity InGaAs/AlGaAs conical LD was studied. The mode changes of the propagation interface were simulated by the eigenmode expansion method (EME). The effects of various separation distances of the beam spoilers were compared and verified by experiments. The results show that through the practical design of the separation distance of beam spoilers at the mode propagation interface, the tapered LD can maintain the high beam quality of M²=1.9 at 3.2 W.

Due to the particularity of the material and structure of the temperature control product ammonia axial channel thermotube on satellite, there are few researches on its forming at home and abroad at present. In the process of product development, defects such as cross section deformation, accuracy out of tolerance and springback will occur, which will reduce the quality and pass rate of the product and affect the progress of the product, laser induced bending forming method is adopted in this paper, and the technological parameters of laser induced bending forming are studied according to the bending radius and angle required by the designer.In actual research process, firstly, we utilize ABAQUE software to simulate the effect of the selection of process parameters and the forming mechanism of laser induced bending forming as well as observing the laser effect in the process of stress, strain and temperature changes as well as angle, referring to the result of simulation test. Finally, we get specific process parameters under bending radius and bending angle.

In this thesis, we designed and experimentally demonstrated a high-power high-speed underwater optical wireless communication (UOWC) system with wavelength conversion construction. External modulation based on 1064nm laser is used for high-speed information communication, as well as the optical amplifier is used to obtain enough optical power of 1064nm laser. After that, according to the quasi-phase-matching (QPM) conditions, the PPLN optical structure is designed to improve the wavelength conversion efficiency for achieving higher 532nm laser output power in 24.5℃~40°C. Compared to the 532nm LD modulation system, this system can output 1.4W 532nm laser power in 100Mbps. This system experiments in single link 100m water tap of the attenuation coefficient 0.73dB/m equivalent to the clear ocean, and the measured bit error rate (BER) is 6.2×10^-6 in 100Mbps pseudo-random binary sequence (PRBS) data without the forward error correction (FEC). Based on receiver sensitivity and the seawater channel optical transmission model, the transmission performance was predicted to be 340m@100Mbps and 100m@2Gbps in the attenuation coefficient equivalent to pure seawater.

Passively mode-locked optically pumped vertical external cavity surface-emitting lasers (OP-VECSELs) have unique properties, such as excellent beam quality, high-average output power and high repetition rate. Many applications, including frequency comb and supercontinuum, require pulses in the femtosecond regime. A numerical model in the femtosecond regime is essential to understand the pulse formation mechanism. In this paper, we present a numerical model of passively mode-locked VECSELs in the femtosecond regime. We analyze the influence of gain parameters, such as small-signal gain, saturation energy, and the gain bandwidth on the pulse characteristics. The results of the simulation can provide a direction for designing the gain chip.

Fabrication inside transparent materials using femtosecond laser has important applications in optical storage, microfluidic devices, and other fields. However, the manufacturing efficiency and quality are limited due to the Rayleigh-length of Gaussian beam and the saturation effect in multi-pulses processing. In this paper, plasma distribution and morphology inside PMMA was modulated by adjusting the focus depth (0~ 200 μm) of femtosecond laser single pulse, which realized the fabrication of modification lines with controllable length and high efficiency. Ultrafast pumpprobe imaging system was used to investigate the evolution of filaments and shockwave inside PMMA during processing. The results showed that the filaments and shockwave inside PMMA transformed from the combination of hemispherical shockwave and filaments into only filaments gradually. Besides, it was observed that the length of filaments extended from 50 μm to 325 μm with the increasing focus depth. The simulation of finite-different time-domain was executed and the results agreed well with the observation results. It indicated the changed optical field distribution inside PMMA caused by different focus depth resulted in the corresponding changes of filaments and the effects of self-focusing and spherical aberration dominated in the mechanism of filaments extension on both sides. The morphology of structure processed inside PMMA was further characterized by optical microscope. It showed that uniform modification lines can be fabricated by single pulse with all focus depth and the length of which increased from 73 μm to 135 μm. These controllable modification lines have great potential in fields like optical waveguide, optical storage and hologram.

Large-scale waveguide ring resonators are key elements in micro-optical gyroscopes. In this paper, by using the nondegenerate two-photon laser lithography technology, we fabricated a silicon nitride waveguide ring resonator on the SiO₂ substrate with a coupling distance of 600nm. The FSR of the WRR we prepared is 10GHz, the finesse of the WRR is 2.3, and the Q value is 4.5ⅹ10⁴ . The fabrication process shows the ability that femtosecond laser lithography can be processed on an insulating substrate at the nanometer scale.

Aiming at tracking and attacking low altitude, slow and small targets with high power laser, a miniaturized high power laser beam emitting and tracking aiming system is designed to meet the requirements of compact type. In the beam emitting part, the single coude mirror is used to realize the waveguide system of laser beam in pitch direction. The structure is compact, which meets the requirements of miniaturization and reduces the difficulty of realization. In the pointing and tracking part, the tracking dynamic performance is improved, based on the compound-axis tracking strategy and combined with the man in the loop dynamic correction of the coarse tracking residual. The stability of the optical axis of the single coude mirror is better than 5μrad. The field test results show that the tracking accuracy of the system is 6 μrad for balloon and 9μrad for small quad-rotor unmanned aerial vehicle, which verifies the feasibility of the design of beam emitting and tracking system.

Optical lithography or 3D printing operates on multi-dimensional movement at high resolution. Therefore, accurate position of both x-axis and y-axis deserves real-time measurement. Herein, we utilizes a half-external cavity orthogonal self-mixing laser interferometry for determining displacement of two independent targets. Synchronous detection of irrelevant displacements with nanometer accuracy (<10 nm) is realized here. Circularly polarized laser beam is optically-multiplexed into two polarization states for two additional external cavity. The laser system only needs a single-channel optical intensity to reflect the perpendicular displacement without reference path, which is conducive to simplify the acquisition channel and collimation.

Vascular targeted photodynamic therapy (VPDT) is a new targeted therapy that relies on light as a precise spatially and temporally stimulus. An improved understanding of the interaction between photosensitizers, oxygen and light elements is of great significance to improve the outcomes. A VPDT device for animal evaluation is designed to meet the needs on the regulation of light distribution and irradiance on a same sample. The device is composed of a programmable light source and a micro-camera. The irradiance and spatial distribution can be adjusted by the light-emitting diode current and the duty cycle of the digital micromirror. Experiments show that the optical uniformity of the device is better than 96 percent, the dynamic range is up 28,815 to 1, and the position resolution is 7.6 micron, which can be used for animal experiment.

Surface defects of fused silica glass have become the main reason that limits the resistance of fused silica components to laser damage. When the laser enters the scratch, the scratch modulates the incident light field to increase the local light intensity, increase the probability of damage to the optical element, and reduce the damage threshold. In order to study the modulation effect of scratches on the light field, this paper considers radial triangular scratches and uses the finite difference time domain (FDTD) method to establish a laser incident triangular scratch model. As a result, with the increase of the scratch width w, the maximum light intensity enhancement factor (LIEF) on the front surface of the scratch is 4.6, and the maximum LIEF on the rear surface can reach 9.2. With the increase of the scratch depth d, the LIEF on the front surface of the scratch is 7.5, and the maximum LIEF on the rear surface can reach 12.2. When the scratch is located on the rear surface, the incident laser occurs totally reflection, the interference effect is obvious, the LIEF becomes larger, and the rear surface is more prone to damage.

Photo-acoustic detection is carried out on the laser-induced damage of fused silica glass. The Nd: YAG laser with the wavelength of 1064 nm, pulse duration of 10 ns and the maximum output energy of 7.0 J is used in the experimental. Results showed that: photo-acoustic could not be detected when damage has not occurred. Photo-acoustic signals containing high frequency components can be received after damage occurs. Compared with the sensor attached to the end surface and placed in the air, attaching the sensor to the side has the characteristics of high signal-to-noise ratio and does not affect the optical path. Above work provide a new technical idea for the photo-acoustic method in the damage detection.

We use the Fresnel vector diffraction integral theory to derive the light through dielectric metasurfaces. The polarization evolution process of a circular Airy vector vortex beam (CAVVB) is investigated on a hybridorder Poincaré sphere. We may realize polarization and phase evolution of the CAVVB along the spherical latitude and longitude.It is found that the metasurface can manipulate the symmetric transmission of the CAVVB, which is derived from the rotational symmetry of the circular Airy beam and optical vortex. The study provides an alternative way for manipulating the vector vortex light field based on metasurface.

In order to achieve real-time polarization imaging, in order to improve the accuracy of target recognition and quickness, and improve the image quality of polarization imaging, this paper puts forward a kind of based on liquid crystal phase variable retarders LCVR and liquid crystal tunable filter LCTF polarization imaging, through LCVR to obtain the fourth stokes parameter in order to achieve full stokes polarization imaging. Points is used after the focal plane method to analyze the image obtained by CCD, focal plane method for points in a super pixels, spacing between units, making different polarization states measurements do not match the sample point to improve and solve problems, to use frequency domain analysis method to improve the polaroid array arrangement, polaroid array has more advantages, effectively improve the measurement of the highest frequency, improve the image quality of polarization imaging.

In this paper, we study the Goos-Hänchen effect at the interface of anisotropic two-dimensional atomic crystal. This optical effect manifests itself as a in-plane shift. The relationship between the in-plane shift and optical axis direction, conductance, and doping concentration of the two-dimensional atomic crystal was established. Furthermore, the study found that the in-plane shifts at the black phosphorus interface are sensitive to relative to the change of the optical axis angle, doping concentration and frequency. The precise measurement of the parameters through quantum weak measurement technology provides important theoretical guidance for the experimental characterization of two-dimensional atomic crystals.

We demonstrate a method based on transverse second harmonic generation (TSHG) for in-situ modal inspection of single nonlinear micro/nanowaveguide. CdS nanobelts (NBs) with width of several micrometers and length of a few hundred micrometers are synthesized by a thermal evaporation process. Transverse second harmonic (TSH) signal is observed as pumping light coupled inside a single CdS NB from both ends simultaneously. As a result of multimode interference, periodic interference patterns are observed along the whole NB. The influence of multimode interaction on TSHG effect inside single nonlinear micro/nanowaveguide is studied both theoretically and experimentally. Using fast Fourier transform (FFT), the TSH patterns are analyzed. The results indicate several particular modes inside the NB, which is in great well with calculated results. The method can be in principle operated in varieties of nonlinear micro/nanowavegudies including ZnO, CdS, CdTe, GaAs and LiNbO₃.

Space gravitational wave detection, extraterrestrial life exploration and high-precision measurement of earth's gravity field have great scientific significance and wide application value. Inter satellite long baseline high-precision laser interferometry ranging technology can obtain high-precision inter satellite displacement variation, which is the key technology of the above space-based scientific engineering. The laser interference system transforms the inter satellite displacement into corresponding interference signals. The above-mentioned scientific plan has put forward extremely high precision, stability and wide bandwidth requirements for laser interference phase measurement. It can be seen that the high-precision laser phase measurement technology has become the bottleneck to further improve the ranging accuracy and stability. This paper researches the progress of high-precision laser phase measurement technology for space-based precision ranging at home and abroad, analyzes the demand of this technology in space-related fields and proposes the key technologies and preliminary schemes to be solved in the research.

The laser deposition manufacturing (LMD) is a free-form metal deposition process, which allows generating a prototype or small series of near net-shape structures. Despite numerous advantages, one of the most critical issues of the technique is that produced pieces have a deleterious surface finish which requires post machining steps. Mechanical machining method such as milling and grinding has been used to improve the surface quality of the laser additive manufacturing components. However, the mechanical machining method has some drawbacks such as tool wearing and narrow-area difficult to machining. In this paper, we demonstrate the capability of continuous wave (CW) in polishing rough surface of additive manufactured TiAl alloy. The surface morphology, microstructures, corrosion resistance, micro-hardness and wear resistance of samples were characterized using a laser confocal microscopy (OM), scanning electron microscope (SEM), electrochemical analyzer, Vickers hardness machine, and wear tester, respectively. Results revealed that the surface roughness more than 16.06 μm could be reduce to less than 1.76 μm through laser polishing process. It was also found that a hardened layer about 600μm was produced on the TiAl alloy surface after laser finishing. The microhardness of the sample was improved about 8% compared with the mechanical milling method.

A complete strong coupling numerical model is proposed to investigate the nanosecond pulsed laser ablation (PLA) mechanism of aviation K4002 superalloy. In the numerical model, the incident pulsed laser energy is considered as the surface heat flux with Gaussian distribution both in time and space. Deformation mesh and mesh reconstruction technique are utilized to track the ablation-induced boundary recession. In a time-step the iterations between the solution of temperature field and the boundary recession continue until the convergence is obtained, so as to realize the complete strong coupling. More over the laser-induced heat flux has always been bonded with the recession boundary, and the temperature-dependent material properties are also taken into account. The experimental research is carried out to validate the accuracy of the model, experimental and simulation results show good agreement in terms of ablation depth. Base on the model the PLA mechanism of K4002 superalloy is studied, the evolution of temperature field, the ablation zone morphology, the heat affected zone, and the recast layer formation are analyzed. The strong coupling effect in laser ablation process is fully embodied in the model, and the simulation is improved effectively, which provides an effective analysis method for the ablation mechanism of pulsed laser. The proposed model is of great significance to help us understand the mechanism of pulsed laser ablation and the optimization of processing parameters for PLA fabrication.

Laser is another major invention of mankind since the 20th century, after atomic energy, computers, and semiconductors. Laser has the characteristics of high brightness, high direction, high monochrome and high incoality, which makes laser detection become one of the main detection systems for short-range target, and is widely used in military and civilian fields. In the military aspect, laser detection and identification of targets is a key technology of laser short-range detection. Laser detection can improve the hit probability to the target such as tank ,because it can provide accurate distance data for gun fire, detection and so on; in civilian use , Laser ranging is a key technology in 3D laser scanning. Improving the accuracy of laser ranging is beneficial to 3D reconstruction and point cloud data acquisition. Therefore, it is important to improve the performance of the range circuit. In order to improve the range measurement accuracy of pulse laser, a pulse laser range measurement receiver circuit is designed. Based on the operating conditions of photoelectric avalanche tube (APD), this paper analyzes the main performance parameters such as APD peak wavelength, minimum detection power, sensitivity, reaction time. APD500-9T has been selected as the photodetector, The voltage feedback APD bias circuit based on TPS40210 has been designed, which provides stable operating voltage for APD. This receiver circuit analyzes the input capacitor and GBP of the op-amp, OPA657 was selected as the amplifier of the amplification circuit and a reasonable circuit structure has been designed.The circuit model was built and the functional simulation has been performed in Multisim. Finally, through experimental verification, the highvoltage bias circuit can produce 95V high-voltage, meeting the APD500-9T operation requirements. The receiver circuit can fully receive the echo signal.

According to the theory of optical waveguide transmission, the polarization correlation of mode distribution in optical microfiber was simulated by numerical calculation, and the polarization correlation of the loss caused by single particle pollutant was less than 0.01 dB. The experimental system was set up. When using ordinary laser (polarization extinction ratio is about 16 dB), the result showed that the optical microfiber would maintain the polarization of the light source, and the polarization correlation of the loss caused by the particle pollution was about 1.5 dB; If using depolarized SLD light source (polarization extinction ratio is about 0.2 dB) instead, the relative loss variation of signal light was less than 0.11 dB, which reduced by about 13 times, and the polarization correlation caused by particles attached to the optical microfiber was basically eliminated. So that using depolarized light source could solve the polarization related problem of optical microfiber particle pollution sensing.

Circular polarization shift-keying (CPolSK) modulation maps binary digital signals to the two circular polarization states of the optical carrier, which can be applied to the modulation-demodulation of free space optical communication systems. This paper constructs a new optical communication system that uses vortex beams with different orbital angular momentum (OAM) modes as the optical carriers to multiplex CPolSK modulated signals. And with limited OAM modes, we combine amplitude shift-keying (ASK) modulation and CPolSK modulation to construct a modulation and demodulation system which is similar to quadrature amplitude modulation. In the receiver, a designed reference beam is used to interfere with the signal beam. Then the hybrid modulation signals can be easily converted into the intensity changes that can be directly detected for demodulation. The numerical simulation results show that this modulation system can effectively expand the communication capacity of CPolSK modulation system. And after combining ASK and CPolSK, the system can get better communication performance. It shows a wide prospect in the field of free space optical communication.

In the field of laser engineering, various beam quality criterion has poor scalability, and the description of the laser spot is blurry. Most of the already published spot quality analyses focus on the uniformity of the laser spot, without considering the laser spot features. A generally applicable evaluation system has not been established. Follow standardization thinking, an ideal reference spot model was established, and an evaluation method of laser spot quality was proposed. With substituting simulation and experimental images into calculations, it is confirmed that this method can eliminate the interference from spot picture inherent attributes like the number of pixels, size, and luminance. This method makes the spots of various lasers measurable, especially be appropriate for evaluating semiconductor laser spot, multi-mode fiber spot where the beam quality is insufficient.

The optical fiber temperature sensor based on the coupling mechanism of the miniature pull-tapered dual-core photonic crystal fiber has been designed. The dual-core photonic crystal fiber coupling structure was realized by tapering the two cores of the dual-core photonic crystal fiber transversely. At the same time, the end surface of the fusion taper of the dual-core photonic crystal fiber was plated with a total reflection film, thereby a reflective dual-core micro-fiber pulltaper coupler developed. According to the above-mentioned reflective fiber coupler light guide transmission mode, combined with optical waveguide theory, the coupling mechanism of the dual-core photonic crystal fiber was studied, and the relationship between the structure of the dual-core photonic crystal fiber coupler and its light guide characteristics is studied. The increase of the effective refractive index and the coupling length will cause the centerwavelength of the fiber coupler to shift to the long wavelength direction, and the increase of the coupling diameter will cause the center wavelength of the fiber coupler to shift to the short wavelength direction. Based on the above mechanism, an optical fiber temperature sensor based on a miniaturized dual-core photonic crystal fiber coupling structure was developed, and the temperature response characteristics of the sensor were studied and analyzed through experiments. The experimental results show that the measurement temperature range of the miniaturized dual-core photonic crystal fiber temperature sensor can be from 96°C to 1425°C ,and the temperature sensitivity is 0.024nm/°C. The miniaturized dual-core photonic crystal fiber temperature sensor has the advantages of miniaturization, large range, high sensitivity, and immunity to electromagnetic interference.

In this work, we reported a 2 kW monolithic master oscillator-power amplifier (MOPA) configuration based on a novel constant-cladding tapered-core (CCTC) Yb-doped fiber. The CCTC fiber has a constant cladding diameter of ~400 μm and a varying core diameter along the fiber, with a ~24 μm at both ends and ~31 μm in the middle. This special fiber design can not only effectively suppress the stimulated Raman scattering, but also smooth the thermal load in the fiber. The output performance of this fiber was carefully investigated in a bidirectional pump MOPA configuration with respective co-pump and counter-pump scheme, especially on the aspects of the SRS and TMI. As a result, the TMI threshold is measured at ~870 W and ~1980 W in the co-pump and counter-pump scheme, respectively. The maximum output power of 2023 W is achieved with no sign of SRS in the counter-pump scheme. Before the TMI threshold, the beam quality (M2 factor) remains ~1.4 without any mode distortion, and the M2 factor is measured to be ~1.65 at the maximum output power. These results indicate that the CCTC fiber has great potential to simultaneously mitigate the SRS and TMI effect in the high power operation.

HgCdTe has been shown to be the first semiconductor exhibiting single-carrier multiplication in avalanche photodiodes (APDs) up to gain values larger than 1000 and with close to zero excess noise. These results have opened a new windows for low-flux and versatile imaging. In this paper, we report the latest results on a 50μm pitch 128×128 array HgCdTe APDs with x_cd=0.307 manufactured at SITP. Through optimizing the implantation parameters and annihilation parameters, the designed PIN junction structure could be obtained, and then the performance of APD device was improved. The APDs display a gain of 728 around 10V reverse bias, and the standard deviation of the gain was 18.3% of the mean gain of 113 at 7.8V. The GNDC is less than 100nA/cm² at the bias<9V, but the dark current starts increasing significantly faster than the gain at high bias, and then the device becomes dark current noise limited. The excess noise factor F is less than 1.8 up to gain of 700, and the F factor of 94.75% pixels is less than 1.4 at gain of 126. The Noise Equivalent Photon (NEPh) is 16 photons at gain of 500, and a demonstration imaging was shown.

The HgCdTe avalanche photodiode (APD) with built-in gain mechanism has great application prospects in the field of weak light signal detection. Any dark current will be converted into noise affecting the work efficiency of photodetectors. Therefore, the study of dark current mechanism is an important way to obtain high performance HgCdTe APD. In this paper, the photoelectric detection mechanism of planar junction electron injection HgCdTe APD is systematically studied, focusing on two aspects of structural optimization design and performance improvement. The dependence of device performance parameters on structure parameters is obtained by comparing the simulation results and experimental results. The lower the trap concentration, the smaller the dark current under small reverse bias voltage is, and the higher the operation voltage is. The dark current density of HgCdTe APD array is about 10^-8 A/cm² under small bias voltage. In addition to the material parameters, the dark current is mainly related to the internal electric field, which is affected by the width and doping concentration of the depletion region. The results show that device structure is the major determinant for the performance difference of HgCdTe APD. The structural optimization direction of high-performance HgCdTe APD is to further suppress the local electric field and reduce the dark current by adjusting the process.

Carbon fiber reinforced polymer(CFRP) is a typical hard to machine material. High modulus CFRP is one of the most widely used types in spacecraft structures. In this paper, the ultrafast laser precision cutting technology for two kinds of high modulus materials with different thermal conductivity is studied. The single factor ablation experiments are carried out on two kinds of plates with the thickness of 1mm. The threshold value of the two materials and the influence law of different laser fluence and scanning speed parameters on the entrance cutting size and heat-affected zone are obtained, and the physical mechanism of the above laws is described. The results show that the huge difference in the thermal conductivity of the two materials has a certain influence on the difficulty of processing the material and the selection of process parameters.

We carried out a comparative study on ablation threshold behavior of femtosecond laser twin double-pulse processing of typical transparent material, semiconductors and metallic materials. Based on the change of ablation area with pulse temporal separation (100 fs-15ps) under the same spatially distributed Gaussian beam, influence of pulse-separation on normalized double-pulse ablation threshold (which is normalized to the single-pulse ablation threshold) was demonstrated qualitatively. Special attention paid on the variation characteristics of normalized double-pulse ablation threshold in the sub-picosecond pulse-separation range, as well as its value in pulse-separations comparable or larger than the electron-phonon (or ion) coupling time. We show that the ablation threshold behavior of femtosecond laser twin double-pulse is strongly material-specie dependent, however, can be summarized in to several ones. The difference in ionization and/or deionization mechanisms for the non-metal compared is possibly the physical origins for the contrasted behavior of double-pulse ablation threshold.

We measured the temperature of a single point using a uniform fiber Bragg grating (FBG) for the first time. A quasi- Gaussian temperature distribution pattern was achieved by heating at the midpoint of FBG, then the spectral characteristics, mainly including the central wavelength and amplitude of the resonance peaks were investigated in detail. Results show that the attenuation of each peaks accords well with the theoretical predictions, so the temperature of midpoint can be demodulated by the analysis of transmission spectra.

Lead has good corrosion resistance and sealing property, which is often used in cable lead sealing process and various processes. In the production process, there may be small defects in the lead products, which may affect the quality of the products. Therefore, it is of great economic significance to inspect them to ensure their reliability. Due to the variety of lead products, the use of traditional ultrasonic detection is limited, and laser ultrasound has the ability to detect all kinds of surface samples. However, when the inner defect size of lead products is submillimeter or smaller, the echo signal-to-noise ratio(SNR) is low, so the traditional laser ultrasonic technology is difficult to locate the defects. In this paper, the synthetic aperture focusing (SAFT) technology is applied to the laser ultrasonic detection of lead internal defects. The minimum defect identification ability and resolution ability of laser ultrasonic combined with SAFT are studied, which provides reference for practical application.

Aluminum alloy has the advantages of high strength, corrosion resistance and good toughness, so it is widely used in aerospace, automotive manufacturing, and other fields. Laser processing has the advantages of controllable energy, high energy density, wide range of material adaptation, no contact, almost no cutting force, clean and environmentally friendly, etc. Moreover, the complex features and micro structure functionalization of components can be solved by laser processing. The oblique incidence of the laser will reduce the processing depth of beam waist from the geometric level. And the resulting space vector effect can introduce new processing mechanisms. The processing result could be improved by adjusting the vector effect with important value and novelty. In this paper, the mechanism and forming surface of 7075 be processed by nanosecond laser with different positive inclination angles under the same basic process parameters. It is shown that laser milling can effectively reduce the surface roughness of 7075. The result shows that the when the laser inclination angle (θ) is in the range of 0°~40°, the surface roughness of the machined surface decreases with the increase of θ, and the roughness can be as small as 0.15 μm. When 40° < θ ≤ 60°, the machined surface roughness increases with the increase of θ, but it is still significantly lower than the original surface roughness. The main reason for this phenomenon is that the projected area, shape, and energy distribution of the laser beam waist in the processing area are affected by the laser beam lead angle. The energy distribution affects the material removal mechanism: cladding, vaporization, or sublimation. And under the positive inclination angle, the processed surface is first irradiated and then processed for the second time by the laser beam. The relative posture of laser beam and workpiece is conducive to the discharge of slag, thus improving the processing quality. And pointed out that within the range of experimental conditions, θ = 40° is the optimal inclination angle, because the processed surface has the least undulation and the highest flatness. Moreover, this research has guiding significance for improving the surface quality of laser processing.

With the miniaturization of high-power lasers and the rapid development of high-power spatial light modulators, the laser cleaning technology of marine instruments has received great attention in recent years. In this paper, the pulsed laser with a wavelength of 1064nm is used to generate 5×n parallel laser beams using a spatial light modulator to study the laser cleaning 907 steel of marine instruments whose surface is B-grade corrosion. The laser damage threshold of 907 steel is 2.89J / cm². Under the conditions of pulse laser energy density of 7J / cm², overlap rate of 60%, pulse width of 100ns, and pulse frequency of 100kHz, the surface of 907 steel can be completely cleaned by once. The surface cleanliness reaches the Sa2.5 standard, and the cleaning time per unit-area is inversely proportional to the number of laser beams. The parallel shape-adjustable multi-beam laser based on the spatial light modulator effectively improves the cleaning efficiency and flexibility of marine instruments.

Terahertz waves play an increasingly important role in the areas about astronomical exploration and imaging due to its unique atmosphere sensitivity and space transmission property. In order to realize high sensitive terahertz detection, we develop mesa-type silicon-based blocked-impurity-band (BIB) terahertz detectors. For further verify the influence of device structure parameters on the performance of the BIB detectors, the different doping concentration of absorption layer was fabricated. Moreover, the influence of various doping conditions on the performance of BIB terahertz detector were analyzed. Then the blackbody responsivity and response spectral performance were measured at 4K liquid helium cryogen temperatures. The results show the blackbody responsivity is proportional to the doping concentration of the absorption layer within a range. When the doping concentration increase to a certain degree, the bandgap narrowing effect will become obvious, which results to the impurity band and bottom of the conduction band degeneracy and the function of the device will be invalid. According to the measurement results, when the doping concentration of the absorption layer is 7*10¹⁶cm^-3, the BIB terahertz detector we fabricated has the optimal blackbody responsivity and response spectral performance, and its peak responsivity can reach 15.4A/W. As a further step, the focal plane arrays mesa-type silicon-based BIB terahertz detectors will be designed.

Under the condition of limited laser energy, it is of great significance to understand and effectively control the laser distribution in the laminar flow column to solve the problem of depth capability decline caused by the taper effect of the slit in the thick plate material waterjet guided laser processing. In this paper, the distribution of laser energy in the water column is analyzed by means of simulation and experiment. In the simulation analysis, using ZEMAX software, the energy distribution of axial position and radial position (diameter section) in water column is simulated and analyzed for the different focal length and nozzle diameter. In addition, the simulation results are verified by experiments. The results show that: the laser energy in the laminar water column still presents the characteristics of Gaussian like distribution, which will make the material remove very difficult at the edge of the water column due to the insufficient power density. This is easy to produce machining taper effect. Finally, the energy distribution conditions of high efficiency material removal by water guided laser are given. This study provides a theoretical and technical basis for further understanding the law of energy transmission and distribution of water column in water guided laser.

This paper uses SeeFiberLaser simulation software to simulate and optimize a new type of double-ended output fiber laser. Using the controlled variable method, the reflectance range of the high reflectivity fiber grating is set to 50%-95%, with 5% Step, change the reflectivity of the low reflectivity fiber grating, step by 2%, respectively simulate the laser output power of different reflectivity combinations. The simulation results show that the double-ended laser output and the power can be controlled by changing the reflectivity of the fiber grating. When the reflectivity of the high-reflectivity fiber grating is 50%, and the reflectivity of the low-reflectivity fiber grating is 5%, The total output power of the forward and backward directions is the largest; when the reflectivity of the two fiber gratings are both 50%, the output power of the front and rear ends of the laser is the closest; no matter which direction the fiber grating reflectivity is increased, the output power in this direction will become lower, the output power in the other direction becomes higher and the total output power becomes lower.

Recently, narrow linewidth fiber lasers are wide applied in coherent detection and wavelength beam combining. The influence of fiber grating oscillating longitudinal mode distribution and the amplifier gain fiber characteristics on the output laser spectrum width are explored. The influence of the optical field distribution and the spatial hole burning effect on oscillating longitudinal modes number is analyzed. And the influence of four-wave mixing effect (FWM) in the amplified laser on the spectral width of the output laser is analyzed. The main oscillation power amplification technology of one-stage oscillation and one-stage amplification is used. Finally, the influence of the reflection bandwidth of the resonant cavity, the longitudinal mode number of the main oscillation stage and the length and core diameter of the gain fiber of the amplifier stage on the output laser spectrum width is studied. Spectral width of 0.29nm, power of 2kW and center wavelength of 1064.4nm is realized. The beam quality is M_x²=1.2，M_y²=1.3.

This work presents a novel approach for size-controllable synthesis of silver nanoparticles by ultra-fast pulse laser ablation of silver nitrate aqueous solution with a wavelength of 800 nm and a pulse width of 35 fs. The influence of laser pulse delay and ablated mono-crystal silicon substrate on the silver nanoparticles size distribution was analyzed by means of TEM morphology, SEM morphology, Energy Dispersive X-ray spectroscopy and surface-enhanced Raman spectroscopy. The experimental results show that: 1) As the laser pulse changes from single-pulse to double-pulse and the pulse delay increases from 400fs to 1000fs, the mean size of silver nanoparticles decreases while the average number of nanoparticles increases, making the size distribution more centralized when laser pulse irradiate silver nitrate solution directly. 2) Focusing the laser pulse on the solid-liquid interface, the number of silver nanoparticles produced by doublepulse is several times higher than that of single-pulse. With the increase of pulse delay, the number of nanoparticles increases firstly and then decreases, peaking at 400 fs, and obvious agglomeration occurs on the silicon substrate.

As a new type of micro machining technology, waterjet guided laser processing technology has the advantages of small thermal damage, strong depth ability and small taper effect compared with the traditional dry laser processing technology, and it has broad development and application prospects. 7050 aluminum alloy has been widely used in aerospace manufacturing because of its high strength, high toughness and excellent stress corrosion resistance. In this paper, 7075 aluminum alloy was processed by waterjet guided laser processing technology, and they were studied that the influence of waterjet speed and workpiece feed speed on cutting depth and surface quality. The results show that the cutting depth increases with the increase of waterjet speed in single cutting, and decreases with the increase of workpiece feed speed. By adjusting the process parameters, the surface quality of cutting surface and micro hole machining is better than the results of traditional dry laser processing. In conclusion, the influence of laser processing parameters on the processing of aluminum alloy is analyzed, and the processing method of 7075 aluminum alloy is expanded.

Traditional measurement methods of volume scattering from algae particles can obtain the angular distribution of the scattered light intensity and retrieve the size information of the algae, but cannot be used to analyze the morphological parameters of planktonic algae. Measurement of polarized volume scattering can obtain the influence of the algae particle shape on its volume scattering Muller matrix. To evaluate the effect of morphological parameters on polarization scattering characteristics of planktonic algae particles, a preliminary experimental study was done with the polarization scattering theory of particles in water. The experiment took Isochrysis galbana as the sample and used a linearly polarized laser of 532 nm to irradiated pure sea water and the seawater solution of Isochrysis galbana. To obtain the scattering phase function of the samples under parallel and perpendicular polarization, an optical experiment platform consisting of a polarization state generator (PSG), a sample compartment and a polarization receiving compartment was used to measure the volume scattering of samples. The result shows that, compared with the measurement of pure seawater, the intensity of the parallelpolarized volume scattering from the seawater solution of Isochrysis galbana was significantly increased. Particularly, for backscattering at 120°, the parallel-polarized backscattering of the seawater with algae particles was enhanced. Combined with T-matrix method, a simulation of the polarized volume scattering of Isochrysis galbana was carried out with an approximation of morphological particles and to further verify the influence of the morphological parameters of planktonic algae on its polarization scattering function.

Based on CL51 laser ceilometer data at Daxing Station from October 2016 to February 2017, we analyze the intensity distribution of backscattering at six levels including fog, haze, slight haze, mist, and clear sky, compared the distribution probability of the vertical rate of change of the backscattering intensity of haze and fog. We also compared distribution probability of the average absolute value of the backscattering intensity gradient profile of slight haze and mist. The result shows: the backscattering intensity in the fog layer decreases significantly with the increase in height. In foggy weather, the distribution probability of backscattering intensity is close to clear sky above 300m, the thickness of fog generally does not exceed 300m. Since the particles of the haze layer are evenly distributed, the backscattering intensity in the haze layer changes more slowly with height than in the fog layer, and the thickness of the haze layer is generally greater than 500m. In foggy weather, the absolute value of the vertical rate of change of the backscattering intensity at a height of 0~300m is larger, it is smaller at a height higher than 300m. Since the particles distribution of the haze layer is more uniform than that of the fog layer, in most cases the absolute value of the vertical change rate of the foggy weather near the ground layer (0-100m) is greater than that of the haze weather. There is no obvious difference between the distribution probability of the vertical change rate of the backscattering intensity near the ground layer of slight haze and that of mist. However, in most cases, the absolute average value of the vertical gradient near the ground of mist is significantly greater than that of slight haze.

In this paper, a millisecond laser combined with a nanosecond laser (CPL) was used to interact with the BK7 glass. The influence of the laser fluence and the delay time on the rear surface damage feature was studied. Results show that the damage on the rear surface of BK7 glass was generated by the stress due to the CPL irradiation. Two types of damage morphologies called small damage points (SDP, on the order of hundreds of micrometers) and large damage points (LDP, on the order of several millimeters to centimeters), respectively, were observed. The SDP can be generated by the CPL with low millisecond laser fluence or high delay time, while the LDP showed the opposite feature. The averaged size of the damage point increased with the millisecond laser fluence and dropped rapidly when the delay increased from 600 μs to 800 μs.

Recently, the use of Yb-Raman fiber amplifier has led to a breakthrough in the power of narrow linewidth laser at 1120 nm. In this paper, the effects of seed filtering on the output laser characteristics in a narrow linewidth Yb-Raman fiber amplifier are studied. The experimental results indicate that the seed filtering can not only suppress the ASE noise in the output laser, but also have an influence on the evolution of the sideband noise of the signal spectra. This work can contribute to the understanding of narrow linewidth Yb-Raman fiber amplifiers and can provide a reference for the systematic design.

Cirrus clouds play an important role in atmospheric radiation. Cirrus clouds are mainly composed of tiny ice crystals. Due to the development of industrialization, the ice crystal particles in cirrus may contain some pollutants. Therefore, it is of great significance to study the scattering properties of nucleated ice crystal particles. The research method of this paper is to use Python to write a model file of nucleated long hexagonal prism and flat regular hexagonal ice crystal particles. DDSCAT, an open source software package developed based on DDA, was used to read the model file and calculate the variation of extinction, scattering and absorption efficiency factors with wavelength and effective radius of the two models illuminated by a plane wave. The results show that Q_ext, Q_abs and Q_sca are sensitive to the number of ammonium sulfate nuclei and the shape of ice crystals. The results of this paper are expected to provide theoretical support for the study of cirrus scattering characteristics.

Laser additive manufacturing (LAM) is a novel technology that uses high-energy laser beam to obtain high performance entities and coatings. However, in this process, it is difficult to control the microstructure of materials effectively by changing the laser parameters. Based on this situation, we proposed a method that using direct electrostatic field (DESF) to assist LAM process. In this paper, we investigated the changes in 316L microstructure after assisted by DESF, and discuss the effect of electrostatic field. Microstructures of the 316L stainless steel sample fabricated by this method were tested. The result showed the solidification direction and grain morphology was influenced by the direction and value of DESF obviously. While the ESF direction is opposite to the laser scanning direction, the grains in the longitudinal-section perform orderly growth and the directions are biased to the laser scanning direction. While the DESF direction is the same as the scanning direction, the original solidification direction is obviously changed and the direction tended to be opposite to the scanning direction.

The experiment research regarding ablating the sapphire substrate by picosecond laser with wavelength of 532nm is presented. The influence of adjusted processing parameters on the cracks and clippings via controlling the variable method is studied. Furthermore, the analysis results shows that the thermal effect of 532nm picosecond laser is obvious, cracks and clippings will appear after only a few pulses of ablation. However, when the laser power decreases to 1.32W , the sapphire is ablated, and the cracks and chippings are significantly reduced. Eventually, a straight round hole with a diameter of 0.3mm that have not obvious cracks and chipping would be processed when the energy was selected near the ablation threshold.

Using the Finite-Difference Time-Domain method (FDTD), the properties of the three-dimensional photonic jet generated by the spheroidal particles illuminated by a Bessel beam are studied. The internal and near-field spatial distributions with the change of the ellipticity of the micro-spheroid are investigated for the cases of Bessel beams with different polarizations. The simulation results show that by varying the ellipticity and the polarization of Bessel beam, it is possible to obtain localized photon fluxes having different power characteristics and spatial dimensions. This can be of interest for several applications, such as ultramicroscopy, nanolithography, low-loss coupled resonance waveguides.

In traditional air laser processing of CFRP (cutting), defects such as excessive heat-affected zone(HAZ), fiber extraction, material delamination, and fiber end expansion could occur near the incision. We proposed nanosecond water-assisted laser processing technology and conducted CFRP process research.CFRP has the characteristics of high specific strength, high specific rigidity, light weight and anisotropy. These characteristics make the machining of high-quality CFRP cuts very demanding on parameters, but traditional machining methods always make CFRP defects. The test sample was a CFRP sheet with a thickness of 3mm, which required smooth and even incisions. The test method was to conduct comparative experiments of nanosecond laser grooving in the air and water respectively to test the manufacturing efficiency, the surface roughness of the incision, and the HAZ of inner wall, we also discussed the formation mechanism of notch defects. In comparative experiments, it was found that the quality of the incision was related to the heat accumulation/dissipation ratio. This was related to the single pulse energy, pulse width, and thermal conductivity of the material. In the cut area of CFRP processed in the air, there were phenomena such as thick HAZ, scattered fibers, and slagging. The convective heat transfer of water removes a lot of heat, which significantly reduced the accumulated heat. In general, the water-assisted laser processing could make better cuts. However, it is found that there were some pits near the surface of incisions, which may be caused by the cavitation of tiny bubbles. The shock wave generated by the bubble explosion would not only destroy the fiber structure and increase the residual stress, making the radiation energy distribution uneven, but also brought damages to surrounding materials. We conduct single factor variable experiment at different single pulse energy, overlap rate, and water flow velocity. It is found that under the same frequency, overlap rate and water flow speed, as the energy of a single pulse increased, the HAZ of the material became larger. At a fixed frequency, single pulse energy and water flow velocity, as the moving speed of plate increased, the ablation threshold of the material also increased. It is speculated that it was caused by the scattering effect of small bubbles. The bubbles in the previous pulse could affect the ablation of the next pulse. In the case of small single pulse energy, low overlap rate and high water flow velocity, the influence of bubbles was weakened, and the bubbles in the previous pulse hardly affected the next pulse. Under the conditions of large pulse energy and high overlap rate, there were many large-sized bubbles and the quality of the cut was poor. Under large single pulse energy, high moving speed and high water speed, the cutting quality was the highest and the processing efficiency was high.

Ceramic cultural relics are an essential part of China's historical and cultural heritage. Binders such as epoxy resin adhesive are often used in the repair of ceramic artifacts. After long-term placement, these binders lose their bonding function, so they need to be cleaned so that new binders can be used to continue repairing the ceramic samples. In this paper, laser cleaning technology is combined with the protection of ceramic cultural relics. Try using nanosecond pulsed laser cleaning machine to clean epoxy on the ceramic surface. In this paper, we use laser confocal high power microscope to observe and compare the cleaning effect of adhesive, and explore the best cleaning parameters. According to the control variable method, the three influencing parameters of laser power, scanning speed, and cleaning parameters were changed to control the cleaning degree of epoxy resin. It was observed that when the scanning speed was 500mm/s, the laser power was 35W, and the number of cleaning times was 29, the epoxy resin and contaminants on the ceramic surface were cleaned entirely without damaging the base. When the laser power is increased or the processing times are raised, the removal rate of epoxy resin on the surface of the substrate increases, but the substrate the substrate will be damaged. Therefore, this group of tests is the best combination of cleaning parameters.

The fringe model can directly explain the principle of dual beam differential Doppler effect. Superimposed state vortex light is a petal-like interference fringe superposed by positive and negative topological charge vortex beams. Based on this, the idea of fringe model is applied to the principle of rotational speed measurement based on the rotation Doppler. The rotation Doppler frequency shift based on the model was derived by means of spatial geometric transformation. According to the relative location between the optical axis of vortex and the object rotation axis, three typical incident conditions which are coincident, offset and tilt are considered. The results show that the frequency shift based on the fringe model is consistent with that of the traditional rotation Doppler model, which proves that the model is suitable for the interpretation of the principle of rotation Doppler velocity measurement. This study provides a more intuitive and concise theoretical explanation for the rotational Doppler effect, which has certain guiding significance.